The sentence is bound to generate demands from conservatives for George Bush to issue a politically controversial presidential pardon to a leading architect of the Iraq war.

Libby's punishment, which also includes a $250,000 (£125,000) fine, was at the upper end of expectations. He is the most senior official of any administration in two decades to face a prison term, and - albeit indirectly - the first senior Bush policymaker to be convicted of charges stemming from the war.

As his lawyers pointed out yesterday in a last-ditch plea for leniency, the Vice-President's former top aide was not found guilty of the criminal offence of leaking the name of a CIA operative, Valerie Plame, but only of lying to the grand jury about when he first discussed her with journalists.

However, that argument did not impress federal judge Reggie Walton, who has a reputation as a tough sentencer. "People who occupy these types of positions, where they have the welfare and security of the nation in their hands, have a special obligation to not do anything that might create a problem," he told Libby and his lawyers.

The revelation in a July 2003 newspaper column of Ms Plame's identity came a few days after her husband, the former ambassador Joseph Wilson, publicly accused the Bush White House of deliberately distorting pre-war intelligence about Saddam Hussein's alleged weapons of mass destruction.

The leak was seen as a deliberate effort by the Bush administration to "punish" Mr Wilson. Soon afterwards an independent prosecutor began an investigation that would see a procession of top officials and Washington journalists through the witness box in the DC federal courthouse, a dozen blocks from the White House.

Libby listened calmly as the sentence was read out, but his wife, Harriet Grant, herself a former lawyer, was earlier in tears - and with reason. Even with good behaviour, her husband faces two years behind bars, followed by two years probation. On top of that is a legal bill estimated at $5m. It was "a tragic fall, a tragic fall," Theodore Wells, a defence lawyer, said.

The key uncertainty now is whether Libby will be sent to jail at once, or allowed to remain free pending the outcome of the appeal, which was filed immediately. Judge Walton will rule on that issue next week. His decision could leave Mr Bush facing yet another very awkward decision.

Since their client's conviction, the defence lawyers' strategy has been to keep Libby out of prison and, in American sports parlance, "run down the clock" by spinning out appeals proceedings until Mr Bush reaches the end of his second term. At that point any pardon would have minimal political impact.

But if the judge decides Libby must go to jail at once, the President is bound to come under powerful pressure from conservatives to issue a pardon immediately - risking not only liberal uproar but a possible backlash against his Republican party at the polls. Mr Bush's spokeswoman, Dana Perino, said the President would not intervene so long as the appeals process continued.

Yesterday 160 letters to the judge were made public, many of them from members of the Republican establishment arguing that Libby's record of public service entitled him to the lesser punishment of probation.

Among the letter writers were Donald Rumsfeld, the former defence secretary; General Peter Pace, the chairman of the joint chiefs of staff; the former United Nations ambassador and Iraq hawk John Bolton, as well as the former secretary of state Henry Kissinger.

How an intelligence report led to jail

2002

February: The former ambassador Joseph Wilson travels to Niger to investigate claims that Saddam tried to buy uranium. Wilson reports to the US government that the claims are based on forged documents.

2003

January: President Bush says in his State of the Union address: "The British Government has learnt that Saddam Hussein recently sought significant quantities of uranium from Africa."

June: Lewis "Scooter" Libby, chief of staff to Vice-President Dick Cheney, left, tellsNew York Times reporter Judith Miller that Wilson's wife, Valerie Plame, right, works for the CIA. Other members of the administration leak Plame's identity.

July: Wilson goes public about his Niger trip and says the administration is making up the Niger claims. Seven days later Plame is "outed" as a CIA operative.

Sept: Criminal investigation is launched to track down who leaked Plame's identity.

Oct: Libby is interviewed by Federal agents.

2004

March: Libby testifies to a grand jury and denies leaking Plame's identity.

2005

Oct: Libby is indicted by FBI on five counts: obstruction of justice and two counts each of false statement and two counts of perjury.

2006

Sept: Former US deputy secretary of state Richard Armitage confesses to have leaked Plame's name.

2007

Jan: Lewis Libby's trial for perjury begins.

March: Libby is found guilty on four out of five charges and told to expect a jail sentence.