David Cameron will come under fresh pressure this week to reveal the content of "salacious" messages between him and Rebekah Brooks.
The Prime Minister will be obliged to respond formally to a deadline for a Freedom of Information request from a Labour MP seeking the publication of dozens of private messages between himself and the former chief executive of News International.
So far Mr Cameron has refused to publish all of his correspondence with Mrs Brooks – who faces charges of conspiracy to pervert the course of justice and phone hacking – saying he has given all relevant messages to the Leveson inquiry into press ethics.
However, the Labour frontbencher Chris Bryant, who submitted the FoI request, believes that between 130 and 150 text and email messages between Mr Cameron, his former communications chief Andy Coulson, Mrs Brooks and News International have been kept secret.
Mr Bryant – whose phone was hacked by News International – told The Independent last night that if Mr Cameron refused to release the exchanges he would appeal to the Information Commissioner, Christopher Graham, who has the power to force their publication.
Mr Graham has already made plain that messages sent privately by ministers should be publicly disclosed if they relate to government business, ruling in March that the Education Secretary, Michael Gove, must reveal emails sent via his wife's email account.
Mr Bryant announced his intention to appeal to the Commissioner after a Sunday newspaper published two text messages between Mr Cameron and Mrs Brooks in October 2009, when he was leader of the Opposition.
In one message obtained by the Mail on Sunday, the Conservative leader thanked Mrs Brooks for letting him ride one of her husband's horses, joking: "The horse CB [Charlie Brooks] put me on. Fast, unpredictable and hard to control but fun." He signed the message: "DC".
In the other message, Mrs Brooks told Mr Cameron after his address to the Conservative Party conference: "Brilliant speech. I cried twice. Will love 'working together'."
According to the paper, the messages were supplied to Lord Leveson, which did not publish them on the grounds that they were not relevant to his investigation into the relationship between the press and politicians.
In June, the inquiry published one text message to Mr Cameron from Mrs Brooks – who both had homes in the Oxfordshire countryside – when Mrs Brooks suggested they meet soon for a "country supper". Sent on the eve of his conference speech in October 2009, Mrs Brooks – later arrested by the Metropolitan Police's phone hacking inquiry – told the Conservative leader: "I am so rooting for you too not just as a personal friend but professionally were [sic] definitely in this together! Speech of your life? Yes he Cam!"
There have been allegations that Mr Cameron's government covertly assisted News International's owner, New Corp, in its £7bn bid for BSkyB in exchange for the support of its newspapers at the 2010 general election.
In the Commons, Mr Cameron has persistently refused to give substantial answers to Mr Bryant's questions about the conversations, saying he had given the Leveson Inquiry everything it had requested.
Following the publication of the new messages, Mr Bryant said: "They show that there was a very intimate relationship between them and Rebekah Brooks certainly thought she and David Cameron were working together in a common endeavour. They also reveal that there is more material and whilst every twist of Cameron on this seems to suggest we have seen it all, it is absolutely crystal clear that we haven't."
Saying the 20-day deadline for his FoI request to Downing Street fell this week, Mr Bryant added: "I'm sure these messages will come out one way or another."