27 February 2005 - Fresh evidence has come to light suggesting that Tony Blair committed himself to war in Iraq nearly a year before the American and British assault in March 2003.

The news will heighten the pressure on the Prime Minister to reveal how Britain was drawn into the conflict, in a week when a leading QC has called into question the legal advice on which the Government went to war. Such anxiety is felt in official circles that Special Branch detectives had questioned MPs over leaks, it emerged this weekend.

Downing Street has consistently refused to disclose the date on which Mr Blair promised George Bush that Britain would join the US in an invasion of Iraq. But evidence obtained by the IoS suggests that it was as early as April 2002, when the Prime Minister met President Bush at his ranch in Crawford, Texas.

A ruling by the Parliamentary Ombudsman, seen by the IoS, says the Government sought advice about the legality of a possible invasion of Iraq in the spring of 2002 as the result of "statements made in a particular press release".

The press release is understood to have been in the name of the Foreign Secretary, Jack Straw, who condemned Israel for failing to comply fully with United Nations resolutions calling for it to withdraw after an armed incursion into Palestinian areas. As well as demanding that Israel "respect international law", the press release quoted Britain's then ambassador to the UN, Sir Jeremy Greenstock, who said the "political and moral authority of the UN is not to be cast aside lightly".

The date of the release was 9 April 2002, the day after Mr Blair completed his two-day summit with Mr Bush in Texas. The implication is that immediately after the Downing Street official spokesman had denied that the meeting was a "council of war", the Government was investigating the legality of such a war.

The issue is now being raised by the Liberal Democrats, who are concerned about the sudden urgency of ministers' inquiries immediately after the summit with President Bush. "To be asserting the authority of the UN when there were discussions about possibly breaking the UN Charter is double standards at the very least," said their foreign affairs spokesman, Sir Menzies Campbell. "It underlines the need to know precisely when this request [for legal advice] was made."

Such an early commitment to war in Iraq by Mr Blair, who insisted well into the following year that British military options remained open, would reinforce speculation arising from a book published last week by Philippe Sands QC, a leading international lawyer. It repeats allegations that the Attorney General was "leaned on" to change his legal advice when the war was imminent.

It also emerged this weekend that Special Branch police questioned opposition parties in December about leaked documents on the war. The move to crack down on leaks is thought to be an attempt to prevent the full text of the Attorney General's advice from emerging, as well as further documents relating to the period nearly a year earlier, when Britain and the US were discussing "regime change" in Iraq.

Special Branch detectives interviewed senior staff in the office of Charles Kennedy, the Liberal Democrat leader, and Adam Price, the Welsh nationalist MP, in an investigation ordered by the Cabinet Office into the leaking of highly confidential Foreign Office papers on the war in Iraq.

Mr Price, who has led efforts to impeach the Prime Minister for allegedly lying to Parliament over the war, said he had refused to answer the police questions, believing the approach raised significant constitutional issues about Parliament's independence.

The Plaid Cymru MP said he was told by the police the leak had caused "seething anger at the highest levels".