There is little that can be done to stop politicians avoiding lengthy sit-down interviews, according to TV news chiefs.

Dorothy Byrne, Channel 4’s head of news and current affairs, this week issued a rallying cry to the British television industry, warning that politicians including Boris Johnson and Jeremy Corbyn were adopting the tactics of Donald Trump by declining to appear on major news programmes to subject themselves to scrutiny.

“In evil regimes the first thing they do is arrest or kill the journalists,” she said after her speech at the Edinburgh television festival. “What’s happening here is they’ve not arrested us, they’ve not killed us, they’re trying to ignore us. It’s a similar effect that means they are not being held to account.”

Byrne complained that Johnson – who minimised his media appearances during his leadership campaign and recently began his own online addresses to the nation – has not given a lengthy interview or press conference since becoming prime minister, something that has been contrasted with Margaret Thatcher’s and John Major’s willingness to go on air for lengthy policy discussions.

Katy Searle, the head of the BBC’s Westminster newsgathering operation, said what mattered to her was the overall access given to the BBC as a whole. “The core question is, are they allowing us to hold them to account?” she said.

Searle said the corporation refused to use footage of Johnson’s Facebook broadcasts produced by Downing Street and there was a constant push for longer interviews with the prime minister.

But she said there had to be an acceptance that politicians will only stick their head above the parapet if they have a chance to put their point across – and that there are substantially more outlets than 20 years ago.

“It’s got to work for both sides; if you talk to people in Downing Street they’re not going to put up the prime minister unless there’s a reason to do it. They’ll sit down and ask, ‘what are we going to get out of it?’; this is their message that they need to get across. The journalist understands this and you have to have a grownup view that this is the message they need to represent, while also having the accountability of a grown-up interview.”

Facebook Twitter Pinterest David Frost interviews Margaret Thatcher for TV-am in the 1980s. Photograph: PA Media

Television news, which is heavily regulated, continues to reach millions of people and is substantially more trusted by the British public than other forms of journalism. But its audience is getting smaller and older; while the average over-65 watches more than a half an hour of it a day, those aged 16-24 watch just two minutes.

With declining audiences and television journalists on the hunt for “gotcha” moments that can be clipped and go viral, politicians are increasingly wary of taking part in programmes watched by a few hundred thousand people, with demand for appearances outstripping supply.

Evan Davis, the presenter of BBC Radio 4’s PM, attempted to reimagine the political interview when he joined the late-night Newsnight programme in 2014. In a manifesto written by the programme’s then-editor Ian Katz, they set out to reimagine a way to scrutinise politicians without resorting to the aggressive “Mexican stand-off” format favoured by Jeremy Paxman.

Five years later, Davis accepts that they weren’t entirely successful: “I don’t think it worked, but I don’t think it spectacularly failed. In the end we hadn’t thought through how you make an interview interesting without defaulting to ‘adversarial’.”

He suggested both journalists and politicians are hardwired to the existing format – and that this is the approach that many Newsnight viewers prefer, while he has tried to continue the more probing format on PM following his return to radio: “On any given day it was easier to make it more aggressive. There was a tension with being the post-Paxman presenter, given Paxman was all we understood as a way of making an interview interesting.”

Political advisers on all sides of the divide privately say that while they still care about major news programmes – especially the two main evening bulletins on BBC and ITV, which continue to reach millions of people – they are less bothered about other shows.

There are exceptions – a lengthy cross-examination by Andrew Neil is seen as an unavoidable part of any campaign – but increasingly politicians are asking what is in it for them.

“The Today programme and Channel 4, which certain bits of the liberal political establishment think are very important, we don’t prioritise as much,” said an individual with knowledge of Labour’s press operation.

With only so many hours in the day, political parties are reallocating their candidates’ time towards Facebook and the headlines on commercial music radio, which is listened to by millions of people who may only take a passing interest in politics – rather than set-piece political shows which appeal to people who have already made up their mind.