The prime minister's latest attempt to persuade a sceptical public of his case against Iraq has been undermined by claims of plagiarism against an official dossier on Saddam Hussein's regime.

As Tony Blair faced a critical TV audience on Thursday evening, it was claimed that a dossier of evidence released by the government earlier in the week was largely taken from academic sources.

Channel 4 News said one article used as a source by the document was written on the build-up to the 1991 Gulf War by Ibrahim al-Marashi, a postgraduate student from California.

Glen Rangwala, a lecturer in politics at Cambridge University, told the programme: "The British government's dossier is 19 pages long and most of pages six to 16 are copied directly from that document word for word, even the grammatical errors and typographical mistakes."

Meanwhile, the prime minister has admitted he faces an uphill battle to win over a sceptical public.

But he reiterated his confidence in securing a second United Nations resolution authorising the use of force.

"If there were a second UN resolution then I think people would be behind me," he told the BBC.

"I think if there is not then there's a lot of explaining to do."

However, the latest revelations about the dossier were slammed as a public relations disaster by critics.

"This document has been cited by the prime minister and Colin Powell as the basis for a possible war. Who is responsible for such an incredible failure of judgment?" asked shadow defence secretary Bernard Jenkin.

"We need a clear assurance that the government's published information is based on the best available sources and is not just spin."

And the Liberal Democrats said the news was a further "embarrassment" for the government.

"This is the intelligence equivalent of being caught stealing the spoons. The dossier may not amount to much but this is a considerable embarrassment for a government trying still to make a case for war," said foreign affairs spokesman Menzies Campbell.

But the document was defended by Number 10.

"The report was put together by a range of government officials," said a Downing Street spokesman.

"As the report itself makes clear, it was drawn from 'a number of sources including intelligence material'.

"It does not identify or credit any sources, but nor does it claim any exclusivity of authorship. We consider the text as published to be accurate."

That position was backed by al-Marashi, who told the BBC's Newsnight programme that the dossier "overall is accurate even though there are a few minor cosmetic changes".

"The only inaccuracies in the UK document were that they maybe inflated some of the numbers of these intelligence agencies," he said.