Paul Flynn, the Labour MP behind the group, said yesterday that the All Party Parliamentary Drugs Reform Group will press for a Royal Commission to investigate changes in the law.
The group will also campaign for a private member's bill and Commons debates on the issue. Mr Flynn favours the decriminalisation of cannabis, although not all of his backers agree.
The launch of the group, which will take place in the autumn, and is likely to involve Liberal Democrat MPs as well, will embarrass all three party leaderships. Labour fought a bitter by-election campaign in Littleborough and Saddleworth attacking the successful Liberal Democrat candidate as being soft on drugs.
The Government has consistently taken a tough line against drug use. Paddy Ashdown, the Liberal Democrat leader, clashed with activists at his party conference last year when they backed changes to the law.
Mr Flynn named the three Conservative MPs supporting his group as Alan Duncan, MP for Rutland and Melton, Henry Bellingham, MP for Norfolk North West, and Teresa Gorman, MP for Billericay.
Allies of Mr Duncan, who is now parliamentary private secretary for the Conservative party chairman, said he sympathised with the need for a vigorous debate on the subject but had not yet committed his support.
Labour MPs said to be backing the group include Tony Banks, MP for Newham North West, and William McKelvey, MP for Kilmarnock and Loudoun.
Mr Flynn said: "People believe if you are advocating decriminalisation you are advocating more use. I am emphatically anti-drugs. The last time I took any was in 1974 - and that was two aspirins."Reuse content