Before the questions started about his friend Adam Werritty, Liam Fox had much to celebrate.

The Defence Secretary was credited with bringing at least the beginnings of order to Whitehall's most dysfunctional department, and was claiming for himself a military success in Libya. He was pictured, beaming, with Baroness Thatcher at his 50th birthday party, and there were whispers on the Tory right that he might make a good leader yet. Now, just a few short weeks later, Mr Fox is facing calls for his resignation. He should heed them.

After several days trying to dodge the issue, Mr Fox has at least finally acknowledged that it was "a mistake" to allow "distinctions to be blurred" between his professional and private lives. In a formal statement to the House of Commons yesterday, he reiterated his apologies, shouldered full responsibility for the imbroglio, and stressed his willingness to co-operate with official inquiries into the affair. He also stated categorically that Mr Werritty had had no access to confidential material, had played no role in defence procurement, and had seen no commercial or financial benefit from their relationship.

It was a typically confident performance, and Mr Fox will hope it draws a line under the whole messy affair. But it should not be allowed to do so. While the Defence Secretary's accounts of the controversial meetings in Dubai and Sri Lanka may be deemed sufficient to exonerate him from allegations of actual wrongdoing, they count for very little against the charge that he has exhibited a spectacular and sustained failure of judgement. Even if the narrowly-drawn internal Whitehall inquiries conclude that he has not broken the ministerial code, Mr Fox's oversights should still cost him his job.

The Defence Secretary says that as soon as he realised that his friend was handing out business cards claiming an advisory position that did not officially exist, he asked him to stop. That is all well and good. But Mr Werritty's influence with Mr Fox appears to have been sufficiently convincing to support a reputation among lobbyists as the person to approach to gain access to the Defence Secretary. The 22 meetings in the Ministry of Defence, and almost as many again overseas, will surely have helped, even if they were purely private and social, as Mr Fox maintained yesterday.

As the saga descends ever further into the minutiae of when, where and how meetings were arranged or minuted, the central, still unanswered question, is why Mr Fox failed to recognise that he needed to keep his professional distance from a friend with links to the defence industry once he became the Secretary of State. Such a judgement does not need a ministerial code of conduct, it is plain common sense.

Equally, that Mr Fox attended meetings scheduled by a personal friend and unattended by departmental staff indicates a cavalier and unprofessional attitude that can only be a matter of concern. Mr Fox has admitted that he should not have met a commercial supplier without an official present. But his acknowledgement is too little, too late.

Despite personal tensions that go back to the 2005 Tory leadership contest, the Prime Minister was still trying to save his Defence Secretary yesterday, defending his record at the MoD and maintaining that he had given "a good account of himself" on the subject of Mr Werritty. It is true that Mr Fox has started well at the MoD, not least in tackling the £38bn budgetary black hole. But that cannot outweigh the gravity of his mistakes.

Belated apologies are not enough. Neither are Mr Fox's assurances that Mr Werritty will make no further visits to the department, and will neither go to international conferences attended by Mr Fox nor meet him on overseas trips. If the Defence Secretary does not resign, the Prime Minister should sack him.