British Prime Minister David Cameron said Monday his defence minister had made "serious mistakes" in allowing a close friend to operate as his advisor despite having no official government role.

His spokesman's remarks came just minutes after Defence Secretary Liam Fox issued a defiant account of his conduct in the House of Commons, and after Cameron received the first findings of an inquiry into the scandal.

Those findings revealed that Fox had met Adam Werritty, his former flatmate and best man at his wedding, on 18 overseas visits since taking office in May 2010, and Werritty had visited him 22 times at the Ministry of Defence (MoD).

Fox has been under pressure for days over his ties to Werritty, after claims that the 34-year-old posed as his advisor, brokered meetings with businessmen and accompanied him on ministerial trips including to Sri Lanka and Dubai.

In a statement to lawmakers, the 50-year-old defence secretary said the 18 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.

"Mr Werritty was never present at regular departmental meetings, during private meetings we did not discuss either commercial or defence matters, he had no access to classified documents, nor was he briefed on classified matters," Fox said.

However, he repeated comments he made on Sunday that "it was a mistake to allow distinctions to be blurred between my professional responsibilities and my loyalties to a friend", and said such meetings with Werritty would now end.

Cameron had earlier offered Fox his support while the minister challenges the allegations against him, and said he would wait until the full inquiry by the MoD's top civil servant was completed before making a judgement.

But a spokesman for the prime minister later said the preliminary investigations had raised serious concerns.

"It is clear, as Liam Fox himself said yesterday, that serious mistakes were made in allowing the distinction between professional responsibilities and personal loyalties to be blurred -- and this has clearly raised concerns about impropriety and potential conflicts of interest," Cameron's spokesman said.

He said it was clear that "much tighter procedures" were needed to ensure no future breach of the rules on ministerial behaviour.

And he added that another senior civil servant who reports directly to Cameron would join the inquiry into Fox's behaviour, "addressing all the remaining questions that have been raised by this issue".

The opposition Labour party has been pressing Fox on whether his relationship with Werritty put national security at risk, but the defence secretary received strong support from fellow Conservative MPs in parliament.

He challenged some of the most serious allegations against him, including that he allowed Werritty to set up a meeting in Dubai in June with businessmen hoping to sell phone call encryption technology to the British military.

Fox reiterated that the meeting was the result of a chance encounter between Werritty and one of the businessmen on Dubai.

Although he admitted it should not have taken place without MoD officials present, he said Werritty was not paid for the meeting, nor had he been involved in any defence procurement issues.

He also explained why Werritty attended a meeting he had with the Sri Lankan president in London last year, saying it was a private meeting, and offered a similar defence for Werritty's presence on a visit to the country in July.

Fox is a member of Cameron's Conservative party, although he has more traditional views than the prime minister and they have clashed in the past.

He nevertheless has a key role in the coalition government in pushing through defence spending cuts, as part of its deficit reduction programme, as well as in Britain's military operations in Libya.