The News of the World tried to smear Tom Watson with an untrue story that he was having an affair with a devout Muslim woman, the campaigning MP has claimed.

Journalist Mazher Mahmood, the so-called “fake sheikh” who dressed up as an Arab prince to pursue stories, had Mr Watson placed under surveillance by a private detective for a week.

Mr Mahmood was the News of the World’s star reporter, although far from royalty, he was actually the son of an ordinary couple from Small Heath, Birmingham.

In evidence presented to the Leveson Inquiry into the culture, practice and ethics of the press, Mr Watson, Labour MP for West Bromwich East, said he believed the surveillance was part of an attempt “to smear all 12 members” of the Commons Culture, Media and Sport Committee, which was looking into allegations of phone hacking at the News of the World.

The MP also revealed that he quit his job as a government minister in 2009 at the urging of his wife, who couldn’t cope with the media intrusion into his life.

And he said he had pursued the campaign to expose wrongdoing at the News of the World and its owner News International on the assumption that newspapers would do so much to discredit him that he would become unelectable and lose his Commons seat.

Mr Watson also warned that media abuses including the phone hacking scandal were a result “of the failure of politicians to act in the public interest”.

Many politicians were scared to stand up to the media – and those that did risked having their reputations trashed, he said.

The MP made the comments in a detailed written statement presented to the inquiry, which was commissioned by David Cameron, the Prime Minister, following claims that News International staff engaged in in phone hacking, police bribery and other unethical practices.

Describing attempts to smear his reputation, Mr Watson said: “I was followed by private investigator Derek Webb. Recent disclosure from the company shows that the covert surveillance was commissioned by Mazher Mahmood with someone called ‘Conrad’ acting as an accomplice.

“Mahmood made untrue and damaging claims in internet emails with the subject heading ‘Labour sex scandal’.

“He claimed I was having an affair with a devout Muslim woman. It goes without saying that the allegation alone would have caused immense damage to the reputation of the woman concerned.”

At that point, Mr Watson had come to the attention of News International because of his role investigating phone hacking claims. However, he had previously suffered harassment because of his role in pressuring Tony Blair, the former Prime Minister, to stand down – which had angered Mr Blair’s supporters in the media.

Mr Watson said: “When I stood down as a minister it was because my wife asked me to stand down. The pressure had become too much.

“I was at a crossroads in life. I was done with it and didn’t want to be part of that world any more.”

Mr Watson quit as a Cabinet Office Minister in June 2009. It was only after he joined the Culture, Media and Sport Committee later that year that he began to investigate the phone hacking scandal, he said.

Eventually, he decided to continue his inquiries even if it destroyed his career.

“Before the 2010 general election I made the decision that I was going to get to the facts of the scandal whatever the consequences.”

Mr Watson has also revealed in his book, Dial M for Murdoch, that the strain of pursuing his inquiries led to the end of his marriage.

In the evidence to Lord Leveson, Mr Watson said politicians who fail to follow the media’s agenda risk having their reputations destroyed.

He said: “The more terrified we were of their contempt for us, the more we craved and courted their approval.

“It’s hard not to conclude that the politicians who choose not to moderate their thinking in this way, generally lose out in the political stakes.

“I cite current examples Kenneth Clark, Vince Cable and Chris Huhne, to illustrate a point.”

Mr Huhne is facing trial over charges of perverting the course of justice, in connection to a 2003 speeding case. Dr Cable was the victim of a newspaper sting when journalists posing as constituents recorded him saying he had “declared war” on the empire of Rupert Murdoch, the Australian media tycoon who controls News International’s parent company News Corp.

Meanwhile, MPs have agreed to refer News International executives to the Commons Committee on Standards and Privileges for alleged “Contempt of Parliament”, after the Culture, Media and Sport Committee accused them of misleading it.

Chapter eight of a Culture, Media and Sport Committee report into the affair accuses former News International chief executive Les Hinton, former Legal Manager of News Group Newspapers Tom Crone and former News of the World editor Colin Myler of misleading the committee.

In practice, it is very unclear what would actually happen if the Commons Committee on Standards and Privileges did find that a person was in contempt of Parliament.

In the past, the Commons had the power to order members of the public to come to the Commons and explain themselves, and even to jail them until the end of the Parliamentary session – until the next Queen’s Speech in other words. But according to the Clerk of the Commons, “the House’s power to punish non-Members for contempt is untested in recent times”.