The country's largest household energy supplier has come bottom of an annual league table for customer service as UK households continue to grapple with punitive bills.
The consumer group Which? said British Gas was the worst performer overall after factoring in its own assessment of companies' practices to complement its survey of bill-payers.
The Centrica-owned firm was followed jointly by Boost - part of Ovo Energy - and So Energy.
British Gas received an overall score of 56%.
It got two out of five stars for most categories, including accessibility - or whether a customer can engage with it effectively, value for money and accuracy of energy payments.
It received three stars - an average score - for customer service overall.
British Gas received just over half marks for customer service and just one point out of 10 for performance against its smart meter targets.
It also received four out of 10 for how it handled complaints, losing points primarily for the volume of customer complaints it received per 100,000 customer accounts in the first half of 2023 in comparison to other suppliers.
Boost, Scottish Power, Ovo Energy, Shell Energy, EDF Energy and E.ON Next followed while Octopus Energy, Ecotricity and E (Gas & Electricity) achieved the highest customer scores.
Octopus Energy was the only firm to achieve a five-star rating for overall customer service.
The findings were derived from a survey of just over 9,000 household energy customers in October.
In that month, the energy watchdog Ofgem revealed that it was to further tighten, from December, its expectations for energy companies' treatment of customers, especially those struggling to pay their bills.
Households are grappling with another winter of above average bills following the unprecedented surge in wholesale costs that followed Russia's invasion of Ukraine.
But Ofgem has, this month, allowed a number of suppliers to restart the forced installation of prepayment meters.
Which? director of policy and advocacy, Rocio Concha, said: "With energy prices still punishingly high and limited chances for consumers to save money by switching suppliers, good customer service is more important than ever.
"While Octopus Energy, Ecotricity and E were all named Which? Recommended Providers and scored highly for their customer service, others fell short of customers' expectations.
"Which? is calling for any providers who are falling short on customer service to up their game and ensure customers are able to contact them easily and get the answers they need."
A British Gas spokeswoman responded: "This survey is behind the curve and is based on data that is up to a year old.
"Since that period, we've targeted more than £25m of investment on improving service and customers are seeing a difference - this includes hiring 700 new contact centre agents and extending our opening times.
"We continue to be focused on customer service and helping our most vulnerable customers through our sector-leading £100m customer support package.''