Tessa Jowell, the Olympics minister, defended her estranged husband, David Mills, yesterday as he was sentenced to four and a half years in an Italian jail for taking a $600,000 (£400,000) bribe as a reward for withholding court testimony to help Italy's prime minister, Silvio Berlusconi.

The conviction, which Mills said he would appeal against, ends a three-year trial that nearly derailed the ministerial career of one of Tony Blair's allies.

Jowell, who was culture secretary at the time, admitted signing a document crucial to the receipt of what the court in Milan yesterday ruled was a bribe. She asserted her innocence by separating from her husband after the allegations emerged.

Yesterday the Tories said they would not be making party political capital out of the conviction of the tax lawyer. In a statement yesterday, Jowell stood by Mills, saying: "This is a terrible blow to David and, although we are separated, I have never doubted his innocence."

Mills, 64, was not present in court when the judge, Nicoletta Gandus, read out her verdict, but said in a statement that he was very disappointed by the verdict and would appeal against it. "I am innocent, but this is a highly political case," he said. "I am hopeful that the verdict and sentence will be set aside on appeal, and am told that I will have excellent grounds."

Gandus said Mills would have to pay €250,000 (£220,000) in damages for the perversion of justice caused by his distorted evidence given at two trials in which Berlusconi was a defendant. She also ordered Mills to pay a further €25,000 in costs. Mills's lawyer, Federico Cecconi, told reporters afterwards: "This is a verdict based on a prosecution case that was anything but consolidated. It contravenes the logic and dynamics of the trial."

Berlusconi, indicted alongside Mills, who acted as his legal adviser on offshore dealings, is no longer a defendant. His government passed a law last year giving the prime minister and other top Italian officials immunity from prosecution.

It is thought to be the first time in Italy that someone has been found guilty of taking a bribe without the giver of the money being identified. Gandus's reasons for her verdict will be released in writing at a later date.

Mills is entitled under Italian law to two appeals and the crucial issue now is whether they can be heard before the offence, of which he has been convicted, becomes "timed out" in February next year by a statute of limitations. The prosecutor, Fabio de Pasquale, said: "It is possible, if they get a move on." The fact that Mills failed to appear at his trial was criticised by the judge.

Mills initially admitted having accepted what he had considered a gift or loan. But subsequently he retracted his statement, leaving the prosecution with the formidable task of trying to establish how the money had reached him through a chain of offshore trusts and hedge funds.

In 2000, Jowell and her husband took out a loan, securing it on their terraced house in Kentish Town, north London, and investing the proceeds in a hedge fund. The following month, the loan was repaid with the $600,000 that has been at the centre of the trial.

After the transaction came to light, Jowell said she only became aware four years later, in August 2004, that her husband had received money he "had reasonable grounds to believe was a gift". Tony Blair accepted her assurance.

The issue of Berlusconi's immunity is now being considered by Italy's constitutional court.

The Italian government recently secured Berlusconi's position further with a clause inserted in a bill to reform the judiciary. If approved by parliament, it will mean that, even if such immunity were lifted, the judge presiding over the case that ended yesterday would not be able to apply its outcome automatically to Berlusconi. She would be obliged to begin proceedings all over again.