The Commons could hold a new vote on whether to keep British troops in Iraq if a controversial Bill from Birmingham MP Clare Short becomes law.
The MP for Ladywood, former Secretary of State for International Development, is proposing new laws giving the Commons the right to decide whether British troops are sent into battle.
Her aim was to remove the power of the Prime Minister to commit British forces armed conflicts, and ensure Parliament always had a say.
But a study of the legislation, by Commons officials, has concluded it could come into effect retrospectively - so that Parliament voted on conflicts already taking place, such as Iraq or Afghanistan.
The Bill is supported by senior MPs including Robin Cook, the former Foreign Secretary, William Hague, the former Tory leader, and Tony Wright, MP for Cannock Chase and Labour chairman of the influential Commons public administration committee. But it is likely to be blocked by the Government.
Ms Short came third in the annual Private Members' Bill ballot, which allows her to propose the legislation, and the Bill will be debated on October 21.
The analysis by the Commons library says the effect of the new law would be more wide-ranging than just giving Parliament a chance to vote before the UK goes to war.
A clause giving the Prime Minister the power to make a decision on his own in an emergency - as long as it was later put to a Commons vote - could actually mean new votes took place on Iraq and Afghanistan, the analysis said.
The report says: "Under this clause retrospective approval would have to be granted for the participation of British forces in armed conflicts that begun prior to the legislation coming into force."
Ms Short said: "I never had it in mind, and I don't think the drafters of the Bill had it in mind, for it to have retrospective implications."
It was "interesting and surprising", she said.
"I am sure the Government doesn't want the Bill at all. Whether they will come up front and oppose it, or try to use the whips to undermine it, I would suspect the latter."
She added: "Surely, it is the role of Parliament to hold the executive, where we have a tremendous amount of concentrated power, to better account."