The House of Lords Constitution Committee is to hold an inquiry into the royal prerogative power, which allows prime ministers to deploy the Armed Forces without the backing of MPs.

The inquiry will intensify the pressure on Tony Blair to give Parliament the final say over whether troops go into battle. He broke with precedent in March 2003 and offered MPs a vote before committing troops to war in Iraq, although their decision was not binding.

Since then Gordon Brown, the Chancellor, has said he believes that Parliament should have the final say over whether troops are to be deployed. During the general election campaign, Mr Brown said that he would consider changing the law to give MPs that power if he were to become prime minister.

MPs will also debate the issue soon. Clare Short, the former International Development Secretary who resigned after the Iraq war, has tabled a Private Member’s Bill on stripping the Prime Minister of war-making powers, giving them instead to MPs.

She says that her Bill has the support of 200 MPs from all parties, including senior Conservatives such as Willam Hague and Kenneth Clarke. Ms Short came third in the annual Private Member’s Bill ballot, which gives the Bill a good chance of proceeding through the Commons.

The second reading will be on October 21, and the Bill will be in committee by Christmas. At that point Mr Blair will have to decide whether or not to give her Bill parliamentary time. With the House of Lords committee’s inquiry under way, he will be able to argue that the issue is already being explored in detail by Parliament, and say that he would prefer to await the committee’s conclusions before allowing the Bill to pass.

The power to declare war was historically the prerogative of the sovereign, but it has gradually shifted to the prime minister over time and he or she has no obligation to gain the approval of Parliament.

Lord Holme of Cheltenham, the chairman of the committee, said that war-making powers had become one of the most important constitutional issues of the day and feelings run understandably high. “It is important to conduct a thorough inquiry to identify how the requirements of democracy and national emergency can best be reconciled,” he said.

The committee will examine the alternatives to royal prerogative powers and the models used by other democratic countries. The focus of the report, however, will be on what role Parliament should have and what form that should take.

It will also examine whether the Government should be required to explain its legal justification for war, including providing the evidence on which the legal justification is based.

The legal advice for war in Iraq became a big political problem for Mr Blair. He published a summary of the legal advice, but was repeatedly asked to provide the full legal advice because MPs suspected that it may carry crucial caveats.

The full legal advice was leaked during the general election campaign. It revealed, as expected, genuine concerns that without a second UN resolution the decision to go to war could be challenged in court.

Lord Goldsmith, the Attorney-General, could not assure the Prime Minister that a court would agree with his view that the war was legal.

The constitutional reform group Charter 88 welcomed the Lords’ inquiry. “Going to war is one of the most important decisions a country can take, but our democratically elected Parliament has no formal right to debate the issue, as the prime minister uses the royal prerogative to make the decision,” Ron Bailey, the group’s co-director, said.

“The antiquated royal prerogative is more in tune with the 15th than the 21st century and there is massive support both within Parliament and the public for change. President Bush must ask Congress before being able to deploy US troops abroad, while there is no requirement for the British Parliament to even debate the issue.”

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