Tony Blair was warned by Clare Short weeks before the Iraq war began that the United States was unprepared for running the country after the invasion, newly-released letters have revealed.

Former international development secretary Clare Short alerted the then-prime minister to a "possible humanitarian crisis" unless aid agencies were given more time.

The MP for Birmingham Ladywood said the US body set up to rebuild post-invasion Iraq, the Office for Reconstruction and Humanitarian Assistance, was "under-staffed, under-resourced and under-prepared for the scale of the challenge".

Ms Short wrote to Mr Blair on March 5 2003 - two days before attorney general Lord Goldsmith advised the prime minister the invasion could be legal without a second UN Security Council Resolution, and 13 days before the House of Commons voted in favour of the conflict.

But her previously-secret letter, released by the Iraq Inquiry today, appears to suggest war was already inevitable.

Ms Short wrote to the Prime Minister: "You should be aware that the US and the international humanitarian community are not properly prepared to deal with the immediate humanitarian concerns."

She went on: "A little more time would make the US much better able to deal with some of the humanitarian consequences of conflict.

"My department is doing what we can to advise the UK military on preparations for delivering humanitarian assistance, including in the initial absence of the UN and most international NGOs (non-governmental organisations).

"We too could also be better prepared given more time."

Ms Short also used her letter to Mr Blair to caution that reconstructing Iraq without an explicit UN mandate would breach international law.

She wrote: "My understanding is that the US has not accepted all our arguments on the UN role.

"Unless they do, DfID (the Department for International Development) could do no more than support UN humanitarian efforts, and few others would be willing to engage."

Ms Short resigned as international development secretary nearly eight weeks after the invasion on March 20 2003 and became an outspoken critic of Mr Blair's handling of the conflict.

Ms Short also warned Mr Blair before the invasion that budget constraints meant DfID could not help to rebuild Iraq without financial guarantees.

She wrote in a letter of February 14, 2003: "I am happy to prioritise Iraq from my contingency reserve, but I cannot take resources from other poor and needy people to assist post-conflict Iraq.

"Without some understanding on finance, I cannot responsibly commit DfID to the exemplary partnership with MoD (Ministry of Defence), which we discussed."