Liam Fox is facing renewed political pressure after emails and video footage emerged appearing to contradict his explanation of his working relationship with a close friend.
Video revealed Adam Werritty, who has no official role, attended a meeting with the Sri Lankan president with the defence secretary.
And emails suggest a meeting with Dubai businessmen was not a chance encounter, as claimed.
David Cameron has demanded answers.
The defence secretary is facing a Ministry of Defence inquiry into his working relationship with Mr Werritty, and the preliminary findings are to be examined by the prime minister on Monday.
Shadow defence secretary Jim Murphy called on Liam Fox to make a statement in Parliament.
He said: "There's questions about the ministerial code and whether it has been breached, about the professional relationship between Liam Fox and the former best man at his wedding, and a very murky business about links and access to information, that I think is very unsettling."
Deputy Labour leader Harriet Harman said: "It's not just the questions of national security and access to highly sensitive information, it's also the issues of probity and procurement.
"There's now a further question about Dr Fox's integrity."
The MoD investigation will examine whether he broke the ministerial code over his working relationship with Mr Werritty, 34, a former flatmate and best man at Mr Fox's 2005 wedding, by giving him access to the MoD and allowing him to go on official foreign trips.
Video images appear to show Mr Werritty at a meeting Mr Fox had with Sri Lankan president Mahinda Rajapaksa in a London hotel last year.
The MoD said the meeting had been a private one and no special advisers had been in attendance.
The video footage adds to the pressure Mr Fox has been under over revelations that Mr Werritty made regular visits to see him at the Ministry of Defence.
Mr Werritty is also said to have brokered a meeting with businessmen in Dubai as Mr Fox was returning from a visit to British troops in Afghanistan in June.
Mr Fox has previously said in a parliamentary answer: "Mr Werritty is not an employee of the MoD and has, therefore, not travelled with me on any official visit."
At the meeting the defence secretary and the businessmen reportedly discussed technology that allows service personnel to make encrypted phone calls.
On Saturday, email correspondence published by the Guardian appeared to call into question Mr Fox's explanation it had been an impromptu meeting, rather suggesting Mr Werritty had been involved in planning the discussions for some time.
Mr Fox had said defence industry representatives had asked for the meeting in Dubai "when they happened to be sitting at a nearby table in a restaurant".
The Guardian website said the emails were sent between Mr Werritty and Harvey Boulter, chief executive of private equity company Porton Group.
One from Mr Boulter, dated 16 June invited Mr Werritty and his "boss" to his home.
Mr Boulter told the Guardian: "The fact that a meeting was going to happen was pre-arranged in April.
"A meeting with the MoD doesn't happen by chance. I'm sure I wouldn't have just got to meet him (Mr Fox) unless I'd been pre-briefed."
Mr Werritty also used to carry business cards which said he was an adviser to Mr Fox, despite having no official capacity within the MoD or the Conservative Party.
Labour is demanding Mr Fox makes a full statement in the House of Commons.
Liam Fox has responded to the controversy by saying he has "absolutely no fear of complete transparency in these matters".
He told the Sunday Telegraph: "I think there are underlying issues behind these claims and the motivation is deeply suspect."
The ministerial code requires ministers to ensure there is no conflict between their public duties and private interests.
But Jim Murphy told the BBC: "We can't just continue with this drip, drip, drip of allegation and half answer. We need to get to the truth of this.
"There's issues about whether Mr Werritty had access to privileged information. He was a regular visitor to the MoD HQ in London.
"We're told that he didn't travel abroad with the secretary of state on official business, but it now appears that he was certainly was in those countries, those towns, those meetings at exactly the same time as the secretary of state."
"This individual is handing out business cards claiming to work for the secretary of state and yet he wasn't on the MoD payroll or the House of Commons payroll. There's a series of allegations here and we need full answers."
Speaking on the Andrew Marr show, Harriet Harman urged the prime minister to deal with the matter promptly and make a statement on Monday: "People expect the secretary of state to tell the truth, that's the absolute bottom line."
The MoD said that Mr Werritty "never has been part of Dr Fox's official travelling party when the secretary of state is abroad on official business".
A spokesman said: "Mr Werritty's meetings with the secretary of state at the MoD have concerned entirely private matters, not to discuss MoD business.
"At no time has he had access to any classified MoD documents or information."