David Cameron's former independent adviser on ministerial interests has said he should have been asked to look into claims that Liam Fox's friendship with his self-appointed adviser Adam Werritty broke the ministerial code.

Sir Philip Mawer, who has since stood down from the post, told the public administration select committee on Tuesday: "I do believe the adviser ought to have been brought in to investigate the Fox/Werritty affair and ought to have been brought in quickly...

"I was frustrated at the failure to call me in on the Fox/Werritty affair, and I made clear to the Cabinet Office at the time I felt the credibility of the independent adviser had not been assisted by the decisions that were taken," he said.

But Mawer was clear that the incident had nothing to do with him stepping down, saying: "It was not the cause of my decision to leave."

Mawer, added: "If I’d have been engaged, if I'd have been asked to investigate, I would not have restricted myself simply to looking at the question of Fox’s interest, narrowly defined. Clearly, I would have had regard as to the Code as a whole."

In October Fox resigned as defence secretary following allegations that he allowed close friend Werritty improper access to the highest level of government affairs.

The Cabinet Secretary, Sir Gus O'Donnell, was tasked with looking into claims that Fox broke the ministerial code by allowing Werritty to accompany him on 18 overseas trips since the election.

Shadow defence secretary Jim Murphy called for Mawer to look into the allegations surrounding Fox shortly before the minister resigned.

Mawer was succeeded in the post by Sir Alex Allan, who also appeared before the committee. He told MPs he would step down if he felt he was "being deliberately bypassed in favour of the Cabinet Secretary doing investigations."

"I have had a brief discussion with Jeremy Heywood, Sir Gus O'Donnell's successor, about the issue, and making the point that there are advantages to him of bringing the adviser in early and whenever major issues arise, and he accepted the point," he said.