David Cameron threatened to close down the BBC in comments made to journalists on a campaign bus during the election, according to the corporation's political editor, Nick Robinson.

Robinson said it was hard to know if the comments, which colleagues who were on the bus relayed to him, were a joke or a veiled threat. But the journalist, who also came under sustained attack from Alex Salmond during the Scottish referendum campaign, says they have had a real impact on the organisation and added to the pressure felt by BBC staff.

Cameron's remarks came as senior Tories piled pressure on the BBC during the election campaign over its coverage and also commented on the broadcaster's future as it approaches negotiations over the next BBC charter, Robinson says.

Speaking on The Andrew Marr Show on Sunday, Tony Hall, the BBC director general, said there was 'broad agreement' that the licence fee model of funding the BBC could be reformed but that it had at least "10 years of life in it". Under fire from the government over costs, he also indicated that changes due to be announced later this summer would help to "simplify" the BBC.

In an interview with the Guardian, Robinson said he had "thought quite a lot" about comments Cameron made during the campaign, which were interpreted by BBC staff as a veiled threat and "another bit of pressure", since first writing about them in his recently released book about May's election.

Robinson said his sources on the Tory bus said Cameron had responded to a BBC story about something he allegedly told his deputy, Nick Clegg, calling it "rubbish", before going on to say: "I'm going to close them down after the election."

Though Robinson has written that it was impossible to know if Cameron's comments were a threat or joke, he said the impact on BBC staff was real.