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Parliament's sleaze watchdog has launched a formal probe into John Whittingdale over a trip to Amsterdam he took with his dominatrix ex-girlfriend.

The Culture Secretary was reported after a Mirror investigation for not declaring a two-night, expenses-paid trip visit to the MTV Awards on the register of MPs' interests.

A source close to the top Tory said he did not need to because the trip's cost did not meet a £660 threshold - coming just under it at £530.

But he faced questions after it emerged he had declared a similar trip in 2006, and cheapest available pairs of tickets for the ceremony alone were selling for £160.

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In a complaint to Parliamentary Commissioner for Standards Kathryn Hudson, Labour MP Neil Coyle wrote: "I understand accommodation, drinks, food, tickets for the awards and return flights were all paid for by MTV for both Mr Whittingdale and his guest.

"It seems very unlikely that this did not exceed [the threshold]."

A spokesman for the Commissioner told the Mirror she has now launched an investigation into Mr Coyle's claims.

A full investigation launch does not mean an MP has broken the rules, but the Commissioner only chooses some cases to push further.

Earlier this week Ms Hudson declined to probe claims David Cameron had broken Parliamentary rules by not declaring shares in his late dad's offshore fund.

If MPs are found to have broken the rules, a committee of other MPs can then force them to apologise publicly or suspend them from the House of Commons.

Mr Coyle told the Mirror: "It's good to know no one is above the rules and even Cabinet members will be investigated where there are suspicions they've failed to declare trips abroad appropriately."

We revealed Mr Coyle's complaint last week after the Culture Secretary confirmed he had a six-month relationship with a sex worker who had a 'dungeon' for hire.

Mr Whittingdale said he met the woman on match.com and ended the relationship when he found out her profession.

The claims prompted Labour to say he should step aside from press regulation because four newspapers, including one owned by Trinity Mirror, which owns this website, knew about the story.

Campaigning MP Chris Bryant claimed influential minister had the 'sword of Damocles' hanging above his head when declining to implement parts of the Leveson press inquiry.

The pair's trip to the MTV awards in 2013, where stars included Miley Cyrus, Katy Perry and 1D, happened when Mr Whittingdale was chairman of the Culture, Media and Sport select committee.

(Image: Getty)

Rules at the time said MPs had to declare overseas visits by them or their partner, relating to their position in the House, where the cost was more than 1% of their annual salary.

At the time an MPs were paid £66,396, so they had to declare trips worth more than £663.96.

Source close to John Whittingdale: "John inquired about the cost of the trip from the organiser and it was advised it was £534.82.

"John was told this was under the threshold and so he did not need to declare it."

The register is designed to avoid potential conflicts of interest where MPs vote on legislation which affects an individual or a company that has provided them with donations, gifts or hospitality.

(Image: PA)

There is no suggestion that Mr Whittingdale has any such conflict of interest.

It comes after Mr Whittingdale said his faith in a free press had been "tested to the utmost" by revelations about his private life - but he would not launch a revenge crackdown.

Another of his ex-girlfriends, former soft porn star Stephanie Hudson, claimed he showed her Cabinet papers and texted her a picture from Chequers in an alleged security breach.

And it emerged Mr Whittingdale had visited a lap-dancing club with two MPs on what he insisted was a "fact-finding mission" , but which was not declared in the culture committee's records.

He said yesterday: "Having had my faith perhaps tested to the utmost, I still believe that press freedom is a vitally important component of a free society and we should tread very carefully."