Mr Howard said his party withdrew their support because of the limited nature of the inquiry. Lord Butler’s committee will focus on structures and systems rather than individuals.

In a letter to the Prime Minister, Mr Howard expressed his dissatisfaction on what he regards as a narrower interpretation of the inquiry’s terms of references than the two leaders had agreed to. "Lord Butler has chosen to interpret his terms of reference in what I regard as an unacceptably restrictive fashion," he said.

The withdrawal of Conservative participation will cast doubt on the legitimacy of the inquiry which has now become a one party affair. The Liberal Democrats refused to support the inquiry from the outset.

However, when challenged a prime ministerial spokesman said the inquiry would continue regardless.

"This is an independent inquiry looking independently at the issues and it has a broad-based membership. Whether others choose to be a part of it or not is a matter for them, but this inquiry will continue," he said.