The prime minister's office has consistently refused to disclose the date on which Blair promised US President George Bush that Britain would join the US in an invasion of Iraq.
However, evidence obtained by the UK newspaper Independent on Sunday suggests that it was as early as April 2002, when Blair met Bush at his ranch in Texas.
A ruling by the parliamentary ombudsman 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 newspaper said. The paper said it has seen the ruling.
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, the newspaper said.
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".
"To be asserting the authority of the UN
when there were discussions about possibly breaking
the UN Charter is double
standards at the
Sir Menzies Campbell,
Liberal Democrats spokesman
The date of the release was 9 April 2002, the day after Blair completed his two-day summit with 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 opposition party, 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," their foreign affairs spokesman, Sir Menzies Campbell, said.
"It underlines the need to know precisely when this request (for legal advice) was made."