LONDON — A damning assessment of Prime Minister Blair’s lack of leadership in Iraq amid its descent into lawlessness has been made by one of Britain’s most senior diplomats.

The former ambassador to the United Nations and the first British envoy to Iraq, Sir Jeremy Greenstock, said the prime minister had taken his “eye off the ball” in the crucial first days and weeks after the invasion.

In the starkest language concerning the failure of the government to anticipate the insurgency, Mr. Greenstock said, “In the days following the victory of 9 April [2003], no one, it seems to me, was instructed to put the security of Iraq first. To put law and order on the streets first. There was no police force. There was no constituted army except the victorious invaders. And there was no American general that I could … establish who was given the accountable responsibility to make sure that the first duty of any government — and we were the government — was to keep law and order on the streets. There was a vacuum from the beginning, in which looters, saboteurs, the criminals, the insurgents moved very quickly.”

His trenchant remarks in a BBC documentary came to light as:

• Mr. Blair announced the phased withdrawal of British forces with the return of 1,600 troops of the 7,100-strong deployment.

• The Daily Telegraph learned that British troops will remain until 2012 — much later than had been anticipated.

• The British withdrawal was seized on by President Bush’s opponents in Washington as evidence that the coalition was crumbling.

The British defense secretary, Des Browne, is expected to confirm today that Prince Harry will fly with his regiment to Iraq in April. He will be the first royal to serve in a war zone since the Duke of York flew helicopters in the Falklands conflict in 1982.

The powerful intervention of Mr. Greenstock will dismay no. 10 because he was by the prime minister’s side at the United Nations when Britain failed to secure a second resolution authorizing the invasion.

Speaking in a BBC documentary “Blair: The Inside Years, A Man With a Mission,” Mr. Greenstock said Blair “was tearing his hair from time to time at things not being done more quickly. He wanted a new police force up and running within months. He felt we would go through a dark period, we would have some difficulties but we would break out and come right.

“But he did not focus enough on the means of delivering what he wanted. He did not quite concentrate enough on the instruments for delivering the final result that was needed in Iraq, and that is perhaps where he took his eye off the ball.

“One of the reasons why the input of the prime minister to the president was not fully used … was that it was not the White House making the tactical decisions. It was the Pentagon under Donald Rumsfeld who may have heard the advice, but they made their own decisions.”

The point was underlined by the head of the British army between 2003 and 2006, General Mike Jackson. He told the same program: “The difficulty in a coalition is that Britain was very much the junior partner. It’s a difficult position. You may want to do something differently. But if your large partner cannot or will not be persuaded your way is the right way, you will finish up a bit frustrated. I have no doubt [Mr. Blair] has been frustrated.”

Mr. Greenstock also disclosed how close Mr. Blair came to thinking he would have to resign if he lost the House of Commons vote on the war. Asked if those at no. 10 were packing their bags, he said: “They were examining what was in their drawers to see how easily it was portable. He was on a knife edge.”

[Mr. Blair told parliament that Britain would withdraw almost a quarter of its 7,100 troops in the coming months — and hoped to withdraw more by late summer, the Associated Press reported. Denmark said it will withdraw its 460-member contingent from southern Iraq by August and transfer security responsibilities to Iraqi forces. The Danes serve under British command in Basra. Lithuania says it may withdraw its tiny 53-troop contingent.

The major effect of the British and Danish withdrawals will likely be political, coming as America is increasing its troops in an effort to stem violence in Baghdad. Democratic leaders could use the announcements to pressure Mr. Bush to put forth his own timetable for American troop withdrawals.

Secretary of State Rice played down the British announcement, saying it is consistent with America’s plan to turn over more control to Iraqi forces when possible.

“The British have done what is really the plan for the country as a whole, which is to transfer security responsibility to the Iraqis as the situation permits,” Ms. Rice said in Germany. “The coalition remains intact and, in fact, the British still have thousands of troops deployed in Iraq.”

An American Marine was killed in Iraq Tuesday in fighting in Anbar province, the military said.]