Officials also admitted that chunks of the document — praised by Colin Powell on Wednesday for its “exquisite detail” — were copied word-for-word from an article by a 29-year-old Californian academic.

The sentences were lifted from an article by Ibrahim al-Marishi, an Iraqi-American, in the September edition of Middle East Review of International Affairs. He, in turn, sourced his information to a 1999 book by the former weapons inspector Scott Ritter, who opposes President Bush’s Iraq policy.

Last night the US State Department said that General Powell was aware of the reports. “The British report contained good information. We’ll leave it to them to talk about how it was put together,” a senior official told The Times.

In London, the Prime Minister’s spokesman accepted that it may have been wiser properly to source the material used in the report and said the internet version might be amended to acknowledge its origins. “It was a pull-together of a variety of sources. In retrospect, we should, to clear up any confusion, have acknowledged which bits came from public sources and which bits came from other sources,” he said.

He refused to say who had been responsible for the alleged plagiarism. However, four officials who worked on the report were accidentally named on an early draft. They include Alison Blackshaw, Mr Campbell’s personal assistant. Sources at No 10 privately admit that early in January, Mr Blair’s aides started to panic as it became clear the UN weapons inspectors were not close to finding a “smoking gun”, nor was there any sign that President Saddam Hussein was going to let the inspectors disarm him.

The aides instructed communications staff to draw together evidence that Saddam was obstructing the officials to make that the central plank of their case against him instead.

Along with material on how Iraq was frustrating the inspectors’ work, they included a section on how the Iraqi security services are structured, using information from Mr al-Marashi’s paper and Jane’s Intelligence Review. Mr Blair’s spokesman, attempting yesterday to preserve the authenticity of the remaining sections of the report, some of which were compiled by MI6, said that they had been based on intelligence reports.

He also tried to distance senior aides, including Mr Campbell, from the plagiarised section, saying it had been merely “seen by the relevant people” before it went out.

Labour MPs voiced anger that the Government’s case was built on such apparently flimsy ground.

The former Defence Minister Peter Kilfoyle said: “It just adds to the general impression that what we have been treated to is a farrago of half-truths, assertions and over-the-top spin. I am afraid this is typical of the way in which the whole question of a potential war on Iraq is being treated.”