John Whittingdale broke no rules by not declaring a free trip with his then girlfriend on the register of MPs' interests, a source close to the Culture Secretary insisted after a Labour MP called for a sleaze probe.
Neil Coyle has written to the Parliamentary Commissioner for Standards asking her to investigate whether a two-day visit to the 2013 MTV awards in Amsterdam paid for by the music giant should have been made public.
Mr Whittingdale faced Labour calls to step aside from decisions about press regulation after it emerged he had dated a sex worker - but he was backed by Downing Street and said to have Prime Minister David Cameron's full confidence.
The senior Tory, who is single, said he had been unaware of the woman's occupation and had broken off the relationship after six months in 2014 when he discovered someone was trying to sell the story to the press.
He insisted the events had no bearing on any decisions he took in his present role - which he began after the relationship finished - after campaigners for tougher press regulation questioned why he had apparently softened the Government's line.
Mr Coyle said the minister "looks like he had something to hide" as a similar gift had previously been declared by him.
The source however said Mr Whittingdale, then chairman of the Commons Culture, Media and Sport committee, had been told by the organiser that the cost was £534.82.
"John was told that this was under the threshold and so he did not need to declare it," they said.
Although rules now require any such trip valued at more than £300 to be registered, at the time the threshold was 1% of an MP's annual pay, around £664.
Mr Coyle told the Daily Mirror: "This looks like a blatant and frankly outrageous breach of very important Parliamentary rules that could represent a potentially serious conflict of interest.
"I will be reporting him to the standards commissioner for further investigation.
"Mr Whittingdale rightly declared a very similar gift in the past for the same event - but failed to declare this one. If that failure was out of embarrassment over his 'plus one', that's no excuse for potentially breaching Parliamentary rules.
"It looks like he had something to hide, and he needs to consider his position."
The newspaper reported that the stated cost did not include the value of tickets for the event.
Several newspapers are said to have declined to run the story about Mr Whittingdale's private life but it appeared on several internet sites and was then reported by BBC2's Newsnight.
The Daily Telegraph said one unnamed cabinet minister had suggested it was part of an "agenda" against him connected to the renegotiation of the BBC's charter - dismissed as nonsense by the broadcaster.
Tory former chief whip Andrew Mitchell said there was no conflict in his party colleague's position and he was "mystified" as to why he had been "put through the mill in a way really he should not have been".
It was a "debatable point" if he would have been better off informing the PM of his situation when he was appointed to the Cabinet, but was entitled to his private life, he told Newsnight.
"He has been utterly consistent. This story should go away."
He told the programme: "If a chief whip was to take confession from every single middle-aged member of the Parliamentary party who had gone out with a girlfriend or a boyfriend and then discovered that they weren't quite what they thought, he wouldn't be able to do his job of looking after the Parliamentary party and making sure the Government gets its business."
Ex-deputy prime minister Nick Clegg said the Government was "desperately trying to wriggle out" of commitments made to tighten press regulation but insisted that would have been the case whoever was Conservative culture secretary.