British officials interviewed the best man at defence minister Liam Fox's wedding Tuesday to ask why he joined the politician on many overseas trips despite having no government role, a source said.

Adam Werritty, 34, was questioned by civil servants a day after Prime Minister David Cameron said the 50-year-old minister had made "serious mistakes" over his relationship with the younger man.

Initial findings published on Monday showed Fox had met Werritty, who is also his former flatmate, on 18 overseas visits since taking office in May 2010, while Werritty had visited him 22 times at the Ministry of Defence (MoD).

A British government source told AFP on condition of anonymity that civil servants had begun interviewing Werritty on Tuesday. The source gave no further details about the interview or where it was taking place.

Fox has been under pressure for days after claims that Werritty posed as his advisor, brokered meetings with businessmen and accompanied him on ministerial trips including to Sri Lanka and Dubai, despite having no security clearance.

The defence minister addressed parliament on Monday, saying that the foreign trips where he met Werritty "in the margins" included family holidays, and insisted most of their meetings at the MoD were "short, social meetings".

According to a list of meetings released by the MoD on Monday, the pair met during the minister's visits to Singapore, Abu Dhabi, Dubai, Florida, Bahrain, Hong Kong, Israel, Washington and Sri Lanka.

One of the meetings was with US general John Allen, who is now in charge of American operations troops in Afghanistan and the Middle East.

Fox said they did "not discuss either commercial or defence matters, he had no access to classified documents, nor was he briefed on classified matters."

But there have been growing calls for an explanation of how Werritty was able to afford the trips in the first place.

London's Evening Standard newspaper calculated that the flights and hotels would have cost at least 32,000 pounds ($50,000, 36,925 euros) and as much as 100,000 pounds.

Werritty's three known consultancies have only earned him little more than 20,000 pounds in four years, The Times reported.

Businessman Harvey Boulter said that Werritty had been the "route" to setting up a controversial meeting with Fox in Dubai in June, and that he did not know until later that Werritty had no government role.

The meeting was held to discuss the hopes of private equity company Porton Group, of which Boulter is chief executive, to sell what he called "sensitive" phone call encryption technology to the British military.

Boulter said the number of trips Werritty took "does beg the question of how he was being paid," he told the BBC.

"He did disclose to me that he liked to travel first class," he said.

"And I remember a conversation with some of my colleagues, thinking 'wow, the MoD is having budgetary cuts and at the same time their advisers are flying around first class,' and I thought that might be politically dicey."

Cameron's Downing Street office said on Tuesday that an inquiry by the head of the British civil service, Sir Gus O'Donnell, would address the "remaining questions" over the case.

O'Donnell coincidentally announced on Tuesday that he would retire at the end of the year.

A senior lawmaker from Cameron and Fox's Conservative party, Patrick Mercer, meanwhile warned that the scandal was in danger of becoming a "distraction" for the Ministry of Defence.

"It is not helping having the Secretary of State for Defence thoroughly distracted by non-defence issues" at a time when Britain is involved in Afghanistan and Libya, said Mercer.

Cameron has so far stood by Fox, a rival in the Conservative party's 2005 leadership contest.

He has more hardline views than the prime minister and they have clashed in the past, but reports have suggested that Cameron would be wary of alienating the influential right-wing of the party by ditching Fox.