Culture Secretary Jeremy Hunt has told MPs he "strictly followed due process" in the way he handled a controversial BSkyB takeover bid by News Corporation.
He said it was not true that the firm had any "back channel" of influence, when he was ruling on the bid.
His special adviser Adam Smith has quit over contact with the firm that he said "went too far".
But Labour's Harriet Harman said Mr Hunt had been "backing" rather than "judging this bid" and should resign.
She has written to David Cameron demanding an investigation by the independent adviser on ministers' interests into whether the culture secretary breached the ministerial code.
Mr Hunt delivered his statement in a noisy House of Commons, following the publication of emails at the Leveson Inquiry relating to his handling of the takeover.
Labour say these show he fell short in his impartial "quasi-judicial" role in ruling on the company's bid to fully take over broadcaster BSkyB.
But Frederic Michel, head of public affairs at News Corp, has said his references to "JH" in emails were actually shorthand for Mr Hunt's special adviser, Mr Smith.
Mr Michel has said he had no direct contact with the culture secretary, after he assumed responsibility for ruling on the BSkyB bid in December 2010.
Mr Smith said the content and extent of his contact with News Corporation was not authorised by Mr Hunt and he was resigning because his "activities at times went too far" and created the perception that the firm "had too close a relationship with the department".
Mr Hunt took over responsibility for ruling on Rupert Murdoch's controversial BSkyB bid when Business Secretary Vince Cable was stripped of the role, having been secretly recorded saying he had "declared war on Mr Murdoch".
In the "quasi-judicial" role, Mr Hunt had to act with impartiality - but Labour say information that emerged at the Leveson Inquiry into press standards on Tuesday showed he had "fallen very far short" of his duties.
A string of emails suggests there was a steady flow of information from the culture secretary's office to News Corp advisers from June 2010 onwards.
In one, dated 24 January 2011, Frederic Michel told News Corp executive James Murdoch he had managed to get some information on Mr Hunt's upcoming statement to Parliament, adding: "Although absolutely illegal..>!".
The following day Mr Hunt announced his intention to refer the takeover bid to the Competition Commission, but only after first giving News Corp more time to address concerns about "potential threats to media plurality".
James Murdoch said that the "illegal" reference had been a "joke" - but Labour said Mr Murdoch got "the very words that Jeremy Hunt was going to use" ahead of his Parliamentary statement.
Mr Hunt told MPs that his adviser's resignation was a "matter of huge regret" to him, but he said the volume and tone of communications had been "clearly not appropriate in a quasi-judicial process".
Mr Hunt went on to say he had followed due process "with scrupulous fairness throughout" and said Labour's claim that "there was a back channel through which News Corporation were able to influence my decisions" was "categorically not the case".
But Ms Harman said it was clear Mr Hunt had already "made up his mind" about the takeover and he should not have taken on responsibility for ruling on it in the first place.
"Your conduct should have been quasi-judicial but it fell far, far short of that and short of the standards required by your office," she told him.
"The reality is, you weren't judging this bid, you were backing this bid and so you should resign."
Mr Hunt replied that that idea was "laughable" as he had taken a series of decisions that were "against what News Corporation had wanted".
He said it was because he had "expressed some sympathy for the bid" before he was given responsibility for ruling on it that he had "changed the process so that at every stage before I made a decision I got the advice of independent regulators which I carefully considered and followed".
'Shadow of sleaze'
He also said there were "a number of exaggerations" in Mr Michel's emails and "countless examples... of things that simply did not happen".
In noisy exchanges at Prime Minister's Questions, Labour leader Ed Miliband said if Prime Minister David Cameron "can't defend the conduct of his own ministers... he should fire them" and said there was a "shadow of sleaze" over the government.
Mr Cameron said Mr Hunt "has my full support for the excellent job that he does" and accused Mr Miliband of not being able to resist "the passing political bandwagon".
Mr Hunt has asked Lord Justice Leveson to bring forward his appearance at the inquiry, which he had been due to address in May.
He told MPs his adviser's role as a point of contact had been "agreed by the permanent secretary" in his department - something BBC political editor Nick Robinson said might provide the culture secretary with some "cover".
News Corp, which owns the Sun, the Times and the Sunday Times, and has a 39% interest in satellite broadcaster BSkyB, abandoned its bid to take over the remaining 61% of the broadcaster in July 2011, after the phone-hacking scandal emerged.
Its boss, Rupert Murdoch, is facing what could end up being two days of questioning at the inquiry under oath.