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I have spoken to a wealthy backer of Liam Fox who says he and half a dozen others raised funds to pay for Adam Werritty to act as the defence secretary's unofficial adviser - someone who, unlike civil servants, could be relied on to champion support for Eurosceptic, pro-American and pro-Israeli policies.

The donor - who wants to remain anonymous - says that he and those he introduced to Fox do not have defence interests but they do share Fox's ideological leanings. When I asked whether I or the official inquiry could see the accounts to prove this, I was told that "keeping accounts is not one of Adam's strong points".

There is one big problem with this explanation, which is clearly designed to help Fox. Having an adviser outside the rules of the civil service paid for by undeclared donors is almost certainly a breach of ministerial rules.

It also raises one big question. Fox himself said on Sunday that his friend had "defence-related business interests" - which suggests that his trips were paid for not just by ideological supporters but by those with defence interests. No official adviser would be allowed to have any such conflict of interest.

Even though the Cabinet Secretary is now running an official investigation it is the prime minister who is the judge and jury of the ministerial code - as he made clear today at Question Time in Parliament. I'm told that David Cameron spoke to a close friend of Liam Fox's last night and assured him that he would do whatever he could to keep the defence secretary in his post.