Serious mistakes were made by Defence Secretary Liam Fox in his working relationship with friend Adam Werritty, Downing Street has said.
Mr Fox apologised to MPs for the controversy, insisting that at no time was national security put at risk.
In a statement, he said he met the businessman 22 times - more than previously stated - at the Ministry of Defence and 18 times on foreign trips.
He told the Commons these meetings and trips with Mr Werritty would now end.
Mr Werritty, 34, was Mr Fox's best man in 2005 and a former flatmate and also used to carry cards describing himself as an adviser to "the Rt Hon Liam Fox MP".
He also brokered meetings for Mr Fox and accompanied him on foreign trips, apparently holding meetings without civil servants present.
In total, Mr Werritty spent time with the defence secretary on more a third of his overseas visits - 18 out of 48 - since he came to office in May 2010.
Mr Fox, who denies wrongdoing, set up a Ministry of Defence inquiry into his conduct on Friday. He said on Monday Mr Werritty would give evidence to the inquiry soon.
Prime Minister David Cameron has discussed the findings of an interim report on the inquiry with Mr Fox, a Downing Street spokesman said following the defence secretary's statement.
The spokesman said: "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."
He said that while the initial conclusion was that no classified or other defence-related official information was discussed with, or given to, Mr Werritty, it was clear "much tighter procedures" were needed in the department to make sure the Ministerial Code was adhered to.
The prime minister is not expected to make a final decision on Mr Fox's future until he sees the full report due on 21 October.
In his statement to the Commons, Mr Fox revealed Mr Werritty, who he met in 1998, had initially worked as a paid intern in his office and later carried out research work, earning a total of £5,800.
Beyond that, he said there had never been a payment from the government.
He also detailed the meetings involving Mr Werritty that had taken place in Dubai and Sri Lanka.
He told MPs: "I accept that it was a mistake to allow distinctions to be blurred between my professional responsibilities and my personal loyalties to a friend and, Mr Speaker, I am sorry for this.
"I have apologised to the prime minister, to the public and at the first opportunity available, to the House."
He added: "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."
But he added: "I accept, with the benefit of hindsight I should have taken greater care to ensure a more transparent separation of government, party political and private business and that meetings were properly recorded to protect myself and government from any suggestion of wrongdoing.
"Again, I accept my personal responsibility for this."
Mr Fox said Mr Werritty would make no more private visits to the MoD, would not attend international conferences where he was present and they would not meet socially abroad when Mr Fox was on official business.
Other details to emerge during Mr Fox's Commons appearance include:
Shadow defence secretary Jim Murphy said it was clear the defence secretary had "drawn a coach and horses" through the ministerial code rules.
He said "we may never know whether it was arrogance, naivety or hubris which led Mr Fox to this", but the British people expected the highest standards of conduct.
Speaking earlier, the prime minister said Mr Fox had been "a very effective defence secretary", but it was right that an investigation was being carried out.
"I'm sure that we can answer these questions and come through all of this," he said.
"One can't rush these things... there are important elements of natural justice you have to show as prime minister. You've got to give people the time to answer questions, to unearth the information necessary to do that.
"One can't run these things to some sort of pre-ordained media timetable."