The televised Tory leadership debate has been cancelled because Boris Johnson refuses to take part, Sky News has announced.

In a development that could represent a fresh blow to the frontrunner’s hopes of becoming prime minister, the broadcaster said he had “so far declined the invitation” to go head to head with rival Jeremy Hunt – who has accused his opponent of “bottling it”.

A spokesman said: “Sky News has been planning to hold a debate tomorrow between the two remaining candidates in the Conservative leadership election. Jeremy Hunt has agreed to take part but Boris Johnson has so far declined the invitation.

“We stand ready to host a debate tomorrow evening if both candidates make themselves available. Without both candidates, tomorrow's debate will not take place. But we will reissue our invitation for Mr Hunt and Mr Johnson to debate live on Sky News next Monday, July 1.”

Mr Hunt has urged his rival not to be “a coward” about facing public scrutiny. The foreign secretary also called on the BBC to stage a head-to-head TV debate within the two weeks, before ballot papers go out to Tory members.

Biggest lies told by Boris Johnson Show all 5 1 /5 Biggest lies told by Boris Johnson Biggest lies told by Boris Johnson Made-up quote for The Times Johnson was sacked from The Times newspaper in the late 1980s after he fabricated a quote from his godfather, the historian Colin Lucas, for a front-page article about the discovery of Edward II’s Rose Palace. “The trouble was that somewhere in my copy I managed to attribute to Colin the view that Edward II and Piers Gaveston would have been cavorting together in the Rose Palace,” he claimed. Alas, Gaveston was executed 13 years before the palace was built. “It was very nasty,” Mr Johnson added, before attempting to downplay it as nothing more than a schoolboy blunder. PA Biggest lies told by Boris Johnson Sacked from cabinet over cheating lie Michael Howard gave Boris Johnson two new jobs after becoming leader of the Conservatives in 2003 – party vice-chairman and shadow arts minister. He was sacked from both positions in November 2004 after assuring Mr Howard that tabloid reports of his affair with Spectator columnist Petronella Wyatt were false and an “inverted pyramid of piffle”. When the story was found to be true, he refused to resign. PA Biggest lies told by Boris Johnson Broken promise to boss In 1999 Johnson was offered editorship of The Spectator by owner Conrad Black on the condition that he would not stand as an MP while in the post. In 2001 he stood - and was elected - MP for Henley, though Black did allow him to continue as editor despite calling "ineffably duplicitous" PA Biggest lies told by Boris Johnson Misrepresenting the people of Liverpool As editor of The Spectator, he was forced to apologise for an article in the magazine which blamed drunken Liverpool fans for the 1989 Hillsborough disaster and suggested that the people of the city were wallowing in their victim status. “Anyone, journalist or politician, should say sorry to the people of Liverpool – as I do – for misrepresenting what happened at Hillsborough,” he said. PA Biggest lies told by Boris Johnson ‘I didn’t say anything about Turkey’ Johnson claimed in January, that he did not mention Turkey during the EU referendum campaign. In fact, he co-signed a letter stating that “the only way to avoid having common borders with Turkey is to vote Leave and take back control”. The Vote Leave campaign also produced a poster reading: “Turkey (population 76 million) is joining the EU”

The challenger used an article in The Times to criticise his rival’s campaign, calling on him to face more questions from the media. “So don’t be a coward Boris, man up and show the nation you can cope with the intense scrutiny the most difficult job in the country will involve.”

It comes as Mr Johnson faces pressure to answer questions about the police being called out to the home he shares with partner Carrie Symonds. The candidate – considered a near certainty to win the battle for No.10 only a week ago – has also suffered a major poll slump following the loud altercation with his girlfriend.

Photographs emerged of Mr Johnson and Ms Symonds holding hands and smiling as they sit at a picnic table in the countryside, in an apparent bid to counter suggestions that their relationship was in trouble.

The MailOnline website, which obtained the pictures, quoted a "confidante" of Mr Johnson as saying: "There has been so much nonsense touted around about their relationship. The truth is that they love each other very much and want to get married as soon as the time is right. Boris only has eyes for Carrie and she is totally smitten with him."

However, there was some scepticism on social media over whether the images were staged and exactly when the photographs were taken, with some Twitter users pointing out that Mr Johnson's hair appeared considerably longer than in pictures taken on Saturday. There was no immediate response from the Johnson camp to inquiries as to whether the pictures were staged or their date.

Among all voters in the latest Survation survey, 36 per cent backed Mr Johnson and just 28 per cent supported Mr Hunt before the row, but the second survey put Mr Johnson on 29 per cent and Mr Hunt in the lead on 32 per cent. Mr Johnson’s lead among Tory voters has more than halved since the incident.

Mr Johnson repeatedly refused to answer questions about the domestic row during a flustered exchange with Iain Dale at a hustings event in Birmingham on Saturday. “I don’t think they want to hear about that kind of thing,” the former foreign secretary claimed.

On Sunday Conservative MP Liam Fox said: “It’s always easier just to give an explanation … what we can’t have is it being a distraction.”

Mr Hunt sidestepped questions this morning about whether Mr Johnson was a fit person to be prime minister, but said he still needed to earn the trust of voters.

“The way to earn that trust with Conservative Party members and with the country is to subject yourself to scrutiny, to answer questions about what you actually want to do,” he told BBC Radio 4’s Today programme.

“It is very disrespectful to say that you are not going to do any head-to-head debates, any tough media interviews, for the next two weeks,” he added. “I hope the BBC will have a proper debate in the next two weeks. I hope that if Boris doesn’t turn up they will have an empty chair.”

Mr Johnson’s supporters have lined up to defend him over the domestic incident which saw officers out called to the Camberwell home. Jacob Rees-Mogg hit out at “Corbynista curtain-twitchers” over the row.

Tory MP Nadine Dorries said she had spoken to Mr Johnson and Ms Symonds. “I am totally stunned at the level of harassment they have had to endure,” she said, describing the couple as “together, strong and united”.

Responding to this morning’s Sky News announcement, a spokesman for the Hunt campaign said: “Trying to duck debates and run down the clock until after postal ballots have been returned is just cynical and complacent.