David Cameron is under pressure to order an inquiry into claims that Culture Secretary Jeremy Hunt broke ministerial rules in his dealings with Rupert Murdoch's News Corp empire.
Labour has identified what it said were three specific breaches by Mr Hunt of the ministerial code of conduct in his handling of News Corp's £8 billion takeover bid for broadcaster BSkyB.
Meanwhile the Financial Services Authority was reported to be considering an investigation into whether Mr Hunt's office released market-sensitive information to News Corp in breach of City rules.
In a letter last night to the Prime Minister, Labour deputy leader Harriet Harman said he must now refer the case to his independent adviser on ministers' interests, Sir Alex Allan.
The latest storm to rock the Government centres on the release by the Leveson Inquiry into media standards of an explosive 163-page dossier detailing contacts between a News Corp executive, Frederic Michel, and Mr Hunt's office.
Amid noisy scenes in the Commons, Mr Hunt yesterday denied the stream of emails and texts represented a secret "back channel", insisting he had carried out his quasi-judicial role in relation to the News Corp bid with "scrupulous fairness".
However he was forced to accept the resignation of his special adviser Adam Smith, admitting his contacts with Mr Michel had "overstepped the mark" and were "clearly not appropriate" in a such a quasi-judicial process.
Downing Street was clearly hoping Mr Smith's departure would draw a line under the affair, with Mr Cameron insisting that Mr Hunt had "my full support for the excellent job that he does".
However Labour remains determined to keep up the pressure. In her letter, Ms Harman said Mr Hunt had breached the ministerial code by failing to take responsibility for the actions of his special adviser.
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She said he had also failed to give "accurate and truthful information" to Parliament when he claimed last year to have published all documents and details of all exchanges between the Department of Culture Media and Sport and News Corp.
And in a third alleged breach of the code, she said advance details of a parliamentary statement had been passed to News Corp before the Commons had been informed.
"These must now be referred to the independent adviser on ministers' interests as a matter of urgency," she said.
