Silvio Berlusconi was tonight under withering fire from opposition leaders in Italy after a court declared that he had bribed his lawyer, David Mills, so that he could avoid conviction on corruption charges and hang on to "huge profits made from the conclusion of illicit corporate and financial operations".

The judges were giving the reasoning behind their decision in February to sentence Mills, husband of the British Olympics minister, Tessa Jowell, to four and half years in jail for taking a bribe.

Mills was found guilty of accepting a $600,000 (£387,000) bribe in a case in which he had been indicted alongside his former client, who was alleged to have paid the cash.

But since Berlusconi furnished himself with immunity from prosecution after returning to power last year, the court was unable to reach any conclusion with respect to him.

Today's judgment made explicit what was only implicit three months ago: that the money for Mills came from Italy's billionaire prime minister.

In a tacit recognition of the seriousness of the situation, Berlusconi announced he would make a statement to parliament, but did not fix a date. "It is a simply scandalous judgment, contrary to reality as, I am quite sure, will be established on appeal," he said, adding that he would go before parliament "as soon as I have the time" and "say at last what I have for some time thought of certain judges".

The leader of the biggest opposition group, the Democratic party, Dario Franceschini, said: "He should come to say, 'I renounce the privileges [accorded by the immunity act] and submit myself to a trial like any normal member of the public.' "

A statement by the leadership of the smaller Italy of Principles party said Berlusconi's only alternative would be "to resign immediately, because if the prime minister does not have the courage to let himself be tried, he will legitimise in the consciousness of the Italian people the conviction that he is guilty".

But Mario Valducci, a senior member of the prime minister's Freedom People movement, said: "It is time the judiciary went back to being credible.

"Episodes like this merely cause distress and confusion to members of the public, in addition to confirming how necessary it was to approve the [immunity act]."

Mills designed for Berlusconi and Fininvest, the firm at the heart of his business empire, a web of offshore companies. Prosecutors in the 1990s alleged that it was used, among other things, to funnel illegal contributions to the party of Italy's then prime minister, Bettino Craxi. They also claimed Berlusconi bribed members of the revenue police for favourable treatment of his group's tax affairs.

In both trials, Mills was a key witness for the prosecution.

But, said the judges yesterday, he operated as a "false witness, allowing Silvio Berlusconi and Fininvest to go unpunished".

They upheld the prosecution's claim that in 1997, when he was in opposition, the TV and property magnate arranged for Mills to be rewarded for his testimony.

In a statement that he later retracted, the British lawyer endorsed the version of events that he had given to his accountant in London – that he "kept Mr B out of a great deal of trouble he would have been in had I said all I knew".

The judges said part of the payment was to compensate Mills for the roundabout way he channelled the money into his possession, "all the money-laundering operations that he was going to have to carry out to hide, mask, transform [and] hold the sum that was being illegally paid to him".

The manner in which he guided the money along a chain of offshore trusts and companies over a period of three years represented "one of his most refined and criminal money-laundering activities", the judges said.

Mills, who has vigorously protested his innocence, has not gone to jail because under Italian law he can remain free until he has exhausted the appeals process. The offence of which he was convicted is, in any case, expected to be "timed out" by a statute of limitations early next year.