Liam Fox has insisted he is back to "complete normal working mode" in spite of the wave of allegations coming out about his close friendship with Adam Werritty.

Werritty is expected to be interviewed again today by senior civil servant Sue Gray, the head of the Cabinet Office's Propriety and Ethics unit.

Fox told reporters this morning: "I’m continuing to do what is needed at the moment which is that the defence secretary focuses on defence issues."

Labour MP Anas Sarwar has written to the Electoral Commission to ask them to investigate reports of Werritty's wealthy backers after the controversy surrounding Fox took a new twist on Wednesday after someone describing themselves as a backer anonymously offered an explanation for why Adam Werritty accompanied Fox on so many overseas trips.

On Monday Liam Fox revealed to MPs that Werritty, who was best man at his wedding and a former flat-mate, had been on eighteen foreign trips with the defence secretary since the general election, despite Werritty holding no official government position.

Speculation as to how Werritty's travel arrangements were paid for has been going on for the last day, and on Wednesday a friend of Liam Fox has offered an explanation to the BBC.

Speaking to the BBC's political editor anoymously, the so-called 'backer' of Liam Fox tells Nick Robinson that he was one of those who raised funds for Werritty's travel. This, he says, was to ensure that someone "could be relied on to champion support for Eurosceptic, pro-American and pro-Israeli policies."

However this claim appears to only highlight the fact that Adam Werritty was operating as an adviser to Liam Fox, outside of the guidelines relating to special advisers clearly laid out in the Ministerial Code, a set of rules that the Labour Party insists have been broken in several ways by the defence secretary.

Liam Fox already has three special advisers on file, two of whom earn above the threshold at which their salaries can be disclosed.

Special Advisers are paid for by the taxpayer but can conduct political activities, unlike civil servants who must remain neutral.

The revelation that Werritty was being paid by others to act as an "unofficial adviser" to Fox undermines the MoD's claim that he was only present on foreign trips with the defence secretary in a "personal capacity".

It will prompt questions as to whether Fox knew how his friend was financing his foreign trips. Or perhaps more to the point, how the defence secretary could not have known.

The MoD has previously denied that Werritty benefited financially from his relationship with Fox.

"There is not any occasion when any payment has been made or passed. There is no defence relationship or commercial relationship that he [Werritty] has which he has gained from by being a friend of the secretary of state," defence officials said.

But Jim Murphy, Labour's shadow defence secretary,said the BBC report appears to "blow a hole" in Fox's story.

"Political storm clouds are now gathering above Liam Fox's head," he said,

"These allegations appear to blow a hole Liam Fox's story. He appears to have received funding which he hasn't declared and funded Mr Werritty's actions as his unofficial adviser. Mr Werritty has seemingly profited from his association with Mr Fox.

"This appears a clear breach of the Ministerial Code and Parliament's rules. Unless the Defence Secretary can disprove these claims immediately it is hard to see how long this can continue."

While Kevan Jones, a shadow defence minister, said the latest revelations showed that Fox must have known about the nature of Werritty's financial arrangements.

"Liam Fox has one of the most sensitive jobs in government. We don't know whether Adam Werritty attended meetings where confidential meetings were discussed, because there were no civil servants present. This is a very serious accusation."

When asked if Fox should resign, Jones told BBC News, "Well, it's looking increasingly like that."

Fox is under intense pressure after it emerged that Werritty had been handing out Parliamentary business cards describing himself as an adviser to Liam Fox, a practice queried by senior officials at the Ministry of Defence over the summer.

Earlier on Wednesday friends of Fox continued to support the defence secretary at the expense of Adam Werritty. One told The Telegraph that Adam Werritty "pretended he was something he wasn't."

During PMQs David Cameron told the Commons that the Ministerial Code was very clear. "It is for the prime minister to decide who should be sacked." He told the Commons that the inquiry into Fox was underway, and those conducting it should be allowed to get on with their work.

However David Cameron also told MPs that he will consider publishing any details of meetings between Adam Werritty and officals at Number 10, after revelations that one of the PM's most senior advisers, Gabby Bertin, used to work alongside Werrity in a role connected with Atlantic Bridge.