Tony Blair has dismissed criticism of his reshuffle following a clash with Iain Duncan Smith.

The exchange came after senior Conservative MP Nicholas Soames said the prime minister had torn up the British constitution in a "casual and arrogant manner".

Despite intense criticism of his reshuffle Blair said "I stand fully behind the changes that were made".

Amid angry scenes in the Commons, the Tory leader challenged Blair over his decision to appoint Alistair Darling as a part-time secretary of state for Scotland.

"Can the prime minister say how a full time chaos on the roads can be met by a part-time secretary of state for transport?" asked Duncan Smith.

He said one in five trains was now late and "congestion on the roads is growing every day".

"The prime minister thinks that a record of chaos like that can met by having a part time secretary of state for transport," he added.

Mocking the Tory attack, the prime minister said the opposition had had fought the election on a manifesto committed to combining the devolved posts with another Cabinet post.

"We have implemented Conservative policy," said Blair, who went on to defend the government's record on transport.

"We have got billions of pounds of investment going into our transport system in public investment and in private sector investment," the prime minister told MPs.

"But the problem that he has to explain is this. That investment programme was put to this house a short time ago. It was voted against by the Conservative Party."

Liberal Democrat leader Charles Kennedy challenged the prime minister over the claims by Clare Short and Robin Cook that MI6 had told them Iraq did not possess weapons of mass destruction.

Tony Blair rejected their claims arguing that the intelligence put out "absolutely accurately" described the threat posed by Saddam Hussein which was not about the UK but the immediate threat to the region and the wider world.

Kennedy said the statements by the former secretary for international development and the former Commons leader made it vital that there was an independent inquiry.

"Given the seriousness of the charges made does the PM think this can be adequately investigated by a foreign affairs committee that he refuses to meet and an intelligence and security committee that he controls?" he asked.

Blair rejected the charge, saying both committees were "entirely capable of investigating all the facts and finding the truth".