Labour felt it had little choice but to respond with increasing aggression this week to “hallucinogenic” stories about Jeremy Corbyn and cold war records kept about him by the Czech security service.

That decision saw the party leader go on the offensive, accusing rightwing papers of being controlled by billionaire tax exiles, with the party repeating that it planned to hold a media ownership review if it got into power, and sending a lawyer’s letter to a Tory MP over an ill-judged tweet.

The Conservatives, meanwhile, have sought to stoke the row in an attempt to get it picked up by broadcasters – while at the same time trying to pretend they are above the fray by arguing, none too subtly, that it is the party that supports the press and the existing structure of independent regulation.

The turning point came in Derby on Monday, when Theresa May was asked what she thought about allegations that Corbyn may have unwittingly met Ján Sarkocy, a Czechoslovakian spy posing as a diplomat in the late 1980s, which had run first on the front page of the Sun last Thursday.

The prime minister’s remarks, in response to a question from the Sun, that “where there are allegations of this sort, MPs should be prepared to be open and transparent” arguably gave the BBC and other broadcasters some cover to begin delving into the claims, even though they had only added up to Corbyn admitting one meeting with a Czechoslovakian diplomat, whose name he had not recorded, in November 1986.

Labour had attempted to be robust from the off, describing any attempt to claim that Corbyn was “an agent, asset or informer” as “entirely false and a ridiculous smear”. But other rightwing titles were repeating the claims as they took interest in the story and with Tory MPs including the prime minister piling in, the row was threatening to engulf what little story there was and it was felt necessary to contain the story if it was going to be picked up by broadcast media.

It's the new-look prime minister's questions: no questions, no answers | John Crace Read more

Once, such an approach would have been considered high risk. Tony Blair and Gordon Brown wooed the press, long maintaining personal relationships with Rupert Murdoch and Paul Dacre respectively, while in No 10. But personal attacks on Corbyn motivate the party’s supporters, particularly younger voters, who don’t read the Sun, Mail or Telegraph, and who don’t necessarily remember the cold war.

Ed Miliband broke with the Murdoch press in 2011 following the phone-hacking scandal, promising to break up the Sun and Times empire if he was elected. A controversial attack on his Marxist father Ralph, described by the Daily Mail as “the man who hates Britain” led to a furious row with Dacre’s newspaper in 2013. But if both moves were popular at the time, he nevertheless was badly beaten in the 2015 election.

What helped Corbyn enormously was the behaviour of Sarkocy. A paper in his native Slovakia caught up with him, where Sarkocy improbably claimed that Corbyn told him “what Mrs Thatcher would have for breakfast, lunch and dinner” and that Czechoslovakia was responsible for funding Live Aid.

So on Monday night, Labour began by targeting Ben Bradley, a young Conservative MP, threatening to sue him for having tweeted that the Labour leader had “sold British secrets” to Communist spies. Bradley deleted the tweet within minutes of the threat, but lawyers for Corbyn on Wednesday demanded an apology and a payment to charity in an attempt to draw “a red line” against similar social media postings in the future.

The Daily Mail asked Corbyn at an EEF manufacturing employers’ conference on Tuesday about the claims, allowing the Labour leader to accuse the newspaper of “reproducing some nonsense that was written in the Sun” to cheers and applause. An increasingly confident Labour prepared a social media video on Tuesday in which Corbyn went on the attack, arguing the reporting “shows just how worried the media bosses are by the prospect of a Labour government” – although the Mail accused him of issuing “a chilling threat to Britain’s free press”.

Remarkably, as the story was running into the sand, Theresa May shoehorned a reference to the episode at prime minister’s questions, underlining Tory eagerness to keep the subject in the air, joking apropos of nothing that Corbyn loved “blank cheques”, before adding: “Now I know he likes Czechs, but really.”

Immediately after that, the Mail asked a No 10 spokesman about what May thought about the media. The spokesman read out a pre-prepared section of a speech she gave earlier this month, in which she had said: “A free press is one of the foundations on which our democracy is built,” in an attempt to claim a moral high ground.

Labour, meanwhile, sought to press home the advantage, with a confident briefing of its own, arguing that the Sun and Sarkocy’s allegations were false. Corbyn’s spokesman said these had taken on “ever more absurd, even hallucinogenic forms,” repeating that the party wanted to carry out the second part of the Leveson inquiry into press regulation and insisting that its media review would aim to boost diversity in British media, without specifying any details as to how.

In doing so, it risked entrenching an already adversarial relationship with the rightwing press – but the Labour calculus is that, except possibly with older voters, in the social media era that does not matter.