The prime minister is "rapidly becoming a stranger to the truth", Iain Duncan Smith has told MPs.

Challenging Tony Blair on the latest twist in the weapons dossier row, the opposition leader attacked a "culture of deceit and spin at the heart of government".

Alleging that Number 10 was using government scientist David Kelly as a decoy, Duncan Smith called for Alastair Campbell's head.

The prime minister's director of communications was using the "machinery of government as a personal vendetta", he claimed.

"When will the prime minister realise that until he sacks Campbell no-one will believe a word he says anymore?" asked Duncan Smith.

Accusing the Tories of an "opportunism worthy of the Liberal Democrats" Blair said it was for the BBC to say whether Dr Kelly was the source of the weapons claims.

"The foreign affairs select committee have given their opinion as to the source. We have said through the Ministry of Defence that we don't know who the source is. But the BBC are in the position to know who the source is," he said.

But Duncan Smith said the latest controversy was just one of many bearing "all the fingerprints of Alastair Campbell".

"We have had Formula One, Mittal, Hinduja, Robinson, Mandelson, burying bad news and now even the dodgy dossiers," he added.

"Alastair Campbell and the prime minister have created a culture of deceit and spin at the heart of government."

Liberal Democrat leader Charles Kennedy also challenged the prime minister over Kelly's evidence.

"Was the 45 minute warning based in substance and fact or not? The prime minister makes one claim, the expert before the committee makes another. Now which version of these facts is the correct one?" he asked.

Blair rejected claim that Downing Street had doctored the Iraq dossier.

Such claims, he said, were "completely and totally untrue".

But Kennedy refused to give ground. "Doesn't this continuing contradiction underline the need for the prime minister to announce the setting up of an independent inquiry under a judge, time limited, to get to the bottom of this, for public confidence to be restored for once and for all?"he said.

The clash came as all three parties engaged in a final push to enthuse their troops before the recess.

Ahead of an eight day tour of America and the Far East Blair is facing intense and unprecedented pressure both at home and abroad.

Internationally the pressure is mounting on the Labour leader.

Tomorrow he flies to Washington where he will face an awkward round of discussions with George W Bush.

America has distanced itself from intelligence claiming the Iraqi regime was attempting to buy uranium from the African state of Niger.

But speaking in the Commons the prime minister repeatedly defended the government's use of intelligence.

"I stand by entirely the claim that was made last September. The intelligence upon which we based this was not the so called forged documents that have been put to the IAEA," he said.

"It is not as if this link between Niger and Iraq was some invention of the CIA or Britain.

"We knew in the 1980s that Iraq purchased from Niger over 270 tonnes of uranium. It is not beyond the bounds of possibility that they went back to Niger again."