It's time to "think the unthinkable" and consider introducing conscription to ready the country for a potential land war, Britain's former top NATO commander has said.
General Sir Richard Sherriff, ex-deputy supreme allied commander of the military organisation, warned that the UK defence budget is not big enough to expand the armed forces alone.
He told Sky News: "Conscription to most professional soldiers, and I count myself as one, is absolute anathema.
"Britain's armed forces have traditionally and culturally relied on long service volunteer highly professional soldiers with huge experience - and that is really the way we would all want it to go on."
However, given the current global situation and defence funding cuts since the end of the Cold War, he said: "I think we need to get over many of the cultural hang-ups and assumptions, and frankly think the unthinkable."
"I think we need to go further and look carefully at conscription," he said.
His intervention follows comments from the Chief of the General Staff, General Sir Patrick Sanders, who suggested British men and women could face a call-up to the army in the event of a war with Russia.
Russia 'determined to build an empire'
Sir Richard said on Thursday that even if Russia was defeated in the war against Ukraine, it is going to remain determined to rebuild another Russian empire, determined to subjugate Ukraine, and once it's done that, determined to move on to other countries in the former Soviet space, which includes the Baltic states, all members of NATO.
"So there is a real threat to peace in the Euro-Atlantic region - and the way to preserve peace is deterrence, effective military deterrence, conventional and nuclear," he said.
"That means being ready for the worst case, which is war with Russia. So that means our armed forces have got to have the numbers, the capabilities, the logistics, the training needed."
Troop numbers in 'freefall'
He said the current UK numbers are in "freefall", standing at around 74,000, with forecasts going even lower.
"It is not an army that is ready and capable of producing a war-fighting division, which I would argue is the currency of high-intensity conflict," he said.
To get the volunteers needed it would require a huge amount of effort and money, which he said wasn't there.
"So I think General Sanders is absolutely right to be talking about a citizens volunteer army," he said.
"I think now, against all the odds though, is the time to start talking, thinking the unthinkable, and really having to think quite carefully about conscription if we are to deliver the numbers needed."
Finland - which conscripts at 18 - a good model
Asked by Sky's Kay Burley when military training would need to start in that hypothetical scenario, he did not say when, but did point to Finland as a country to look at as a starting point.
"If you are going to think about conscription you need to look widely, and look at other countries like Finland, a country with a very small professional army of about 20,000 - but which can expand its forces to about 280,000 through mobilisation," he said.
"And the way they do it is universal male conscription starting at 18."
Women are encouraged to volunteer, he said. Soldiers who go no further than private will do six months conscription, specialists nine months, officers do 11 months.
They have a reserve commitment up until the age of about 50, 60 for officers, where they are required to go back and do a number of training days every year so they are ready and able to expand those forces, he said.
'Spend now to deter threats'
Major General Charlie Herbert, a military analyst who has served as a senior NATO adviser, said Sir Patrick was trying to "provoke a debate", nationally and within government, about the size of the army and the defence budget.
But also highlighting some of the threats the UK faces now, particularly the emerging threats from Russia.
"We are seeing the possibility of major conflict in Europe once again," he told Sky News.
He said the size of the British Army has halved in the last 30 years - and suggested Sir Patrick was warning it may need to expand quickly in an emergency.
"I think what Sir Patrick and others are saying is, 'Spend now, invest properly in a capable military in order to deter those threats, so that we don't face them in five or 10 years' time'."
He added: "If we fail to invest now we may well pay the consequences."
'What's coming over the horizon should shock us'
"There's a 1939 feel to the world right now," senior Tory MP Tobias Ellwood told Sky News on Wednesday, warning conscription was a possibility.
He said the UK has been "too complacent" and needed to heed Sir Patrick's warning.
"What's coming over the horizon should shock us. It should worry us. We are not prepared," he said.
Last 30 years have gone well - It's going to get difficult
"We've had a couple of decades, three decades or so since the Cold War, life has gone well. It's now going to get more difficult as authoritarian states exploit our timidity, our reluctance to really put fires out."
He added: "Patrick Saunders is saying prepare for what's coming over the horizon.
"There is a 1939 feel to the world right now. These authoritarian states are rearming, there's a risk averseness about the West in wanting to deal with that, and global institutions such as the United Nations aren't able to hold these errant nations to account.
"In fact, the UN is reaching its League of Nations moment unless it's reformed.
"So that's where the world is heading. We need to wake up to that. There is a mindset now of this era of insecurity that we are heading towards, but we are still on a peacetime defence budget of just 2%. That does need to change."
Defence Secretary Grant Shapps in a speech last week said the world is "moving from a post-war to pre-war world" and the UK must ensure its "entire defence ecosystem is ready" to defend its homeland.
Downing Street has ruled out any move towards a conscription model, saying that army service would remain voluntary.