Michael Howard has ended Conservative involvement in the review of intelligence on weapons of mass destruction.

Prompting Jack Straw to accuse the Tory leader of "shameless opportunism", Howard said his party was pulling out of the Butler inquiry.

The opposition claims the committee is set to concentrate more on structures, systems and processes than on the actions of individuals.

Howard said there was "no justification" for such an emphasis in the terms of reference he had originally agreed to.

In a letter to the prime minister, Howard said Lord Butler "has chosen to interpret his terms of reference in what I regard as an unacceptably restrictive fashion".

Despite Howard's move, Conservative MP Michael Mates will continue to sit on the committee in a personal capacity.

Meanwhile, attorney general Lord Goldsmith is to reveal the legal advice he gave on the Iraq war to parliament's independent ombudsman.

Ann Abraham has launched her own inquiry into the government's refusal to make the documents public.