I'm officially a bad liar.
With a blood pressure cuff pumped tight around my arm, straps around my chest monitoring my breathing, and sensors on my fingers to pick up any traces of sweat, veteran polygraph test examiner Don Cargill says he can easily spot the signs I've told him a fib.
I've denied writing the number three on the piece of paper placed underneath my chair, in a simple exercise designed to show how my body reacts to lying. Even with nothing to lose, it's an uncomfortable experience.
But while commonly associated with daytime television programmes like The Jeremy Kyle Show, the use of lie detectors is expanding within the criminal justice system.
And the answers people give could help determine whether or not they can see their children - or even land them back in jail.
In an office above a branch of Carpetright in west London, Mr Cargill carries out private polygraph tests. His clients have included foreign politicians accused of bribery, bodybuilders who want to prove they haven't taken performance-enhancing drugs, and people accused of stealing from their family or being unfaithful to a spouse.
More and more are trying to cheat the polygraph using instructions found online, he says. "There's a lot of techniques they do but we spot 90% of them or more."
I'm asked to jump up and down and open my mouth before my test. Some people have pressed drawing pins into the bottom of their shoes, or even superglued tacks in their mouths, to create a pain response in their brain to distort the chart, Mr Cargill says.
He asks for identification to make sure a stand-in hasn't been sent and carries out other simple tests to spot signs of sleep deprivation or illegal drug use.
A camera is trained on the subject's eyes to make sure they don't cross them or "zone out", while a seat pad is in place to catch out anyone clenching their bottom.
All of these methods have been used to try to cheat the test, Mr Cargill says.
Can lie detectors be cheated?
It is possible to beat the polygraph, says Newcastle University Emeritus Professor Don Grubin, but it takes a lot of practice with the equipment and examiners are trained to spot the signs of anyone trying to trick the test.
Double child killer Colin Pitchfork - who was jailed for life after raping and strangling 15-year-olds Lynda Mann and Dawn Ashworth in Leicestershire in 1983 and 1986 - was recalled to prison in 2021 partly due to concerns he was using breathing techniques in a bid to beat the lie detector.
The Home Office says the polygraph records physiological changes in a person, quoting research from the American Polygraph Association which found deception is accurately detected in 80 to 90% of cases.
Since 2014, probation services have carried out more than 8,800 polygraph tests, while police have conducted more than 4,600, says Prof Grubin, who explains around 60 to 70% result in disclosures - where someone reveals relevant information.
His company, Behavioural Measures UK, has trained and supervised dozens of police and parole polygraph test examiners over the past decade, and like other experts in the field, he doesn't like the term lie detectors.
The technology detects the "cognitive process" (or the brain working harder) when someone tells a lie, he explains.
The "real skill lies in the experience of the examiner", says Mr Cargill, the chief executive of VAST Screening Technologies Ltd and the chairman of the British and European Polygraph Association.
"Nervous reactions are completely different from people telling lies," he says. "Your heart rate physically increases because you're triggering the autonomic nervous system, which triggers a fight, flight or freeze response. You want to run away."
But critics, including University of Northumbria researchers Dr Marion Oswald, a professor of law, and associate professor Dr Kyriakos Kotsoglou, say much of the research is carried out by the industry itself and the accuracy can't be tested in a real-life situation - because it is impossible to verify if someone has told a lie.
They say it is an intrusive "interrogation" technique used to elicit confessions, arguing the polygraph device itself is unnecessary and could be swapped for putting someone's hand on a photocopying machine - a method apparently used by Detroit police and immortalised in cult American crime drama The Wire.
"If you can convince the subject that she or he is being monitored for lies and they believe it, then she or he will disclose more information," says Dr Kotsoglou.
How are lie detectors used?
The results can't be used as evidence in criminal courts but mandatory lie detector tests have been used as a licence condition for sex offenders since 2014, then rolled out to convicted terrorists in 2021 in the wake of the Fishmongers' Hall attack.
They are also used by police and the security services to monitor the small number of terror suspects made subject to Terrorism Prevention and Investigation Measures (Tpims).
There is currently a three-year trial for their use on domestic abuse offenders, while the new Criminal Justice Bill proposes to extend this to convicted murderers who pose a risk of committing a relevant sexual offence on release.
The tests provide "invaluable information we would otherwise not have had about offenders' behaviour which helps us to better protect the public", the Ministry of Justice says.
The Metropolitan Police is looking into using lie detection technology to vet new recruits or root out corrupt officers following a string of damaging scandals, including the cases of Sarah Everard's murderer Wayne Couzens and serial rapist David Carrick.
But "this is still at an early research stage" and there are no imminent plans to use the technology in this way, the force says.
Others are turning to lie detectors to try to prove their innocence.
Kevin Duffy, 70, passed a polygraph test after he was convicted of sexually assaulting a child, but the results weren't taken into account by the judge who jailed him for more than nine years.
His son Ryan Duffy, 44, says: "If there's something that can be used when it's one person's word against another's that can highlight some kind of evidence, why can't it be used?
"If they are prepared to look at it after conviction for single case issues such as, 'are you using public transport, hanging around schools', and relying on data for probation services, then why can't you use it beforehand?"
People can't be sent back to prison for failing a test, but they can face further sanctions, such as stricter licence conditions, and they can be recalled for making disclosures that reveal they have breached licence conditions or indicate their risk has increased.
The information gathered can be shared with police to carry out further investigations, which could lead to charges, while those found trying to trick the polygraph can also be recalled to prison.
A government report last year revealed four convicted terrorists were sent back to jail as a result of lie detector tests - three were recalled after disclosing "risk-related information", while the fourth didn't comply with their polygraph licence condition.
An increasing number of police forces are using the polygraph, which is seen as a useful tool to monitor and assess the risk of people on the sex offenders' register, allowing officers to concentrate stretched resources on those deemed the most dangerous.
At least 14 of the 43 territorial police forces in England and Wales are now using lie detectors, with 14 police officers and 11 staff qualified as polygraph examiners, according to figures obtained by Liberty Investigates.
The data shows 671 polygraph tests were carried out by the 13 forces who provided figures by calendar year, up from 458 in 2018, and a five-year high.
More forces are expected to start using the technology as the College of Policing makes available training through its "polygraph school".
Suspects facing lie detector tests
Testing is "only mandatory by way of conditional caution or a positive obligation of a Sexual Harm Prevention Order or Sexual Risk Order" imposed by the courts, says the National Police Chiefs' Council, and "any police use of polygraph will form part of a wider and detailed risk management plan tailored to the individual concerned".
But the University of Northumbria researchers say a non-statutory regime of testing is being carried out by some forces, including on suspects during criminal investigation.
People arrested on suspicion of committing online child sex offences, for example, could be asked to take a test as part of a risk assessment to determine whether they can have contact with children, including their own.
Figures obtained by Prof Oswald and Dr Kotsoglou, using freedom of information requests, show that at least 228 such polygraph interviews were carried out over six years.
Other responses indicated use in "voluntary" risk assessments of convicted sex offenders, including those who apply for removal from the sex offenders' register, and ambitions to use polygraph testing for more general offences such as violence.
Prof Oswald says: "I think our concern is that should we really in this country be basing really serious criminal justice decisions in a legal system on a scientific technique that is highly contested, to put it mildly, and hasn't, especially in the policing circumstance, been discussed by parliament?"
The College of Policing says: "The College is working closely with the NPCC to establish a Polygraph School so that policing in England and Wales has access to standardised learning and development in the use of the tool that is tailored to their operating environment.
"The College will develop operational advice so that forces using the tool have a consistent basis on which to do so whilst recognising the relevant legal provisions."
What is the future for lie detectors?
The technology has existed in some form for around a century and now Mr Cargill has invented what he calls a "completely revolutionary" Validated Automated Screening Technology (VAST) system - or "polygraph in a box".
Programmed to find out anything from whether someone has massaged the qualifications on their CV to if they're a member of an international terrorist organisation, he says the device is as accurate as any polygraph examiner and is already being used by police in the UK.
But the testing time is reduced to around 20 minutes from three hours and can be used with just 15 minutes training.
After filling out a questionnaire, the subject - attached to the device, wearing headphones and sensors on their fingers and palm - is instructed to silently answer "no" to the questions by a person in a recorded video on the screen. A human then grills them about any responses that indicate deception.
"I call it a truth verifier rather than a lie detector because what it's doing is testing integrity," explains Mr Cargill.
The technology, he says, would be ideal for use in police vetting and he wants to see it rolled out to areas such as Border Force, where officers could verify someone's age or country of origin.