'He thought any jail time was excessive. He did not see fit to have Scooter Libby taken to jail,' said Mr Snow.

He said that even with Mr Bush's decision, Libby remains with a felony conviction on his record, two years' probation, a US$250,000 fine and probable loss of his legal career. 'This is hardly a slap on the wrist,' he said.

With prison seeming all but certain for Libby, Mr Bush on Monday spared the former chief of staff to Vice-President Dick Cheney.

His move came just five hours after a federal appeals court panel ruled that Libby could not delay his prison term. The US Bureau of Prisons had already assigned Libby a prison identification number.

Mr Snow was pressed several times on whether the president might eventually grant a full pardon to Libby, convicted of lying and conspiracy in the CIA-leak investigation. He declined to say anything categorically.

'The reason I'm going to say ... I'm not going to close a door on a pardon. Scooter Libby may petition for one,' Mr Snow said.

'The president thinks that he has dealt with the situation properly. There is always a possibility or there's an avenue open for anybody to petition for consideration of a pardon.' Mr Bush's decision was sharply criticised by Democrats.

Republicans were more subdued, with some welcoming the decision and some conservatives saying Mr Bush should have gone further.

'The president's getting [a] pounding on the right for not granting a full pardon,' Mr Snow said.

Asked whether Mr Cheney had weighed in on the decision to commute Libby's sentence, Mr Snow said: 'I don't have direct knowledge. But on the other hand, the president did consult with most senior officials, and I'm sure that everybody had an opportunity to share their views.'

Mr Bush said: 'I respect the jury's verdict. But I have concluded that the prison sentence given to Mr Libby is excessive.

'Therefore, I am commuting the portion of Mr Libby's sentence that required him to spend 30 months in prison,' he added.

Special prosecutor Patrick Fitzgerald disputed Mr Bush's assertion that the prison term was excessive, saying Libby was sentenced under the same laws as other criminals.

'It is fundamental to the rule of law that all citizens stand before the bar of justice as equals,' he said.

Mr Bush's decision left some Democrats enraged.

'Libby's conviction was the one faint glimmer of accountability for White House efforts to manipulate intelligence and silence critics of the Iraq war,' said Senate Democratic leader Harry Reid. 'Now, even that small bit of justice has been undone.'

House Democratic leader Nancy Pelosi said Mr Bush's decision showed the president 'condones criminal conduct'.

The former operative, Valerie Plame, contends the White House was trying to discredit her husband, a critic of its rationale for invading Iraq.

Mr Bush said his action still 'leaves in place a harsh punishment for Mr Libby'.

Libby was convicted in March, becoming the highest-ranking White House official ordered to prison since the Iran-Contra affair roiled the Reagan administration in the 1980s.

Testimony in the Libby case revealed the unusual steps Mr Bush and Mr Cheney were willing to take to discredit a critic of the Iraq war.