Boris Johnson has reignited the row over who should fund free TV licences for the over-75s, urging the BBC to “cough up”.

The prime minister has signalled that his government will not reverse policy by stepping in to subsidise the benefit, despite the Conservatives’ promise in the 2017 manifesto that it would be protected.

“The BBC received a settlement that was conditional upon their paying for TV licences for the over-75s. They should cough up,” said Johnson, who is attending the G7 summit in Biarritz.

Responsibility for funding free TV licences for the over-75s, which costs an estimated £750m a year, is being handed over to the BBC in June next year, under a deal struck with David Cameron’s government in 2015.

Theresa May repeatedly urged the BBC to continue funding the benefit but the broadcaster announced in June that most over-75s would have to pay for a licence, with only the poorest protected.

In the 2017 general election manifesto, the Conservatives promised to maintain pensioner benefits, “including free bus passes, eye tests, prescriptions and TV licences, for the duration of this parliament”.

The BBC hit back at Johnson, saying: “We’ve reached the fairest decision we can in funding free TV licences for the poorest pensioners, while protecting BBC services.

“If the BBC funded all TV licences for the over-75s it would mean the closure of BBC Two, BBC Four, the BBC News Channel, the BBC Scotland channel, Radio 5 Live, and several local radio stations. It is a matter for the government if it wishes to restore funding for free licences for all over-75s.”

The shadow culture secretary, Tom Watson, said: “This prime minister’s disregard for older people is appalling. He is trying to blame the BBC for his own government’s policy, but this obfuscation will not work. The blame for scrapping free TV licences lies firmly with the government.”

Research from the House of Commons library found that 3m households would lose their free TV licences as a result of the decision. The BBC has said it will continue to provide licences to over-75s who can provide evidence that they claim pension credit, a means-tested benefit designed to help older people.

A No 10 source said: “The government agreed the licence fee settlement with the BBC in 2015. At the time, the director general said it was a ‘strong deal for the BBC’ and provided ‘financial stability’.

“It saw BBC income boosted by requiring iPlayer users to have a licence, and unfroze the licence fee for the first time since 2010 – with it rising each year with inflation. In return, we agreed responsibility for the over-75 concession would transfer to the BBC in June 2020. The BBC must honour this agreement.”