"Obviously, I know nothing about this," Valls said on RTL radio on Tuesday, declaring himself "sad" and "perhaps disgusted" by Cahuzac's statement on the first day of his trial for tax evasion.
The former minister aimed to "sow doubts, including about a man who has just passed away", Valls added.
Rocard, who Valls regarded as a mentor, was the first political leader to allow a vote on transparency in political finance, the prime minister said, insisting on the ethical rectitude of the man who was prime minister from 1988 to 1991.
Secret account was 'for Rocard election campaign'
On the first day of his trial on Monday, Cahuzac told the court that the first bank account opened for him in Switzerland by far-right activist and lawyer Philippe Péninque, was for money to finance Rocard, who had become a leading figure on the right of the Socialist Party, in the 1993 parliamentary election.
"We hoped he would stand" in the 1995 presidential poll, he said.
Cahuzac, who had worked in the health ministry under Rocard's premiership, said the money came from pharmaceutical companies, with two payments from US-based multinational Pfizer, and was banked abroad at their suggestion.
The Mediapart investigative website, which broke the secret accounts story, on Tuesday repeated claims that companies had paid bribes to win contracts to provide scanners to hospitals and clinics while Cahuzac was at the health ministry.
Rocard did not know about the Swiss account, Cahuzac said on Monday, and he refused to name the politicians who did, while saying that some had "progressed" in life.
After the Socialists' "rout" in the parliamentary poll the project was forgotten and nobody told him what to do with the money, Cahuzac told the court, and he later transferred it to another account.
Story changes several times
Cahuzac started paying more money into that account in 2000-2001, this time earnings from his hair implants clinic, and had another Swiss account, opened in 1993, for his earnings as a surgeon abroad.
Cahuzac, who faces trial for tax fraud and money-laundering along with his ex-wife and three other defendants, could face up to seven years in prison and a fine that could be as high as one million euros.
Asked why he had not made the Rocard allegations at the time, he said that he had decided to take all the responsibility himself but had changed his mind because he did not think it would harm Rocard's political heritage.
Sarkozy's supporters condemn trial move
In Tuesday's radio interview Valls also commented on the mainstream right's reaction to prosecutors' calls for former president Nicolas Sarkozy to stand trial in an election campaign finance case.
A spokesperson of Sarkozy's Republicans party, Brigitte Kuster, commented that the justice system was showing "unaccustomed speed" in the case, while his lawyer Thierry Herzog said the move was a "new, crude political manoeuvre".
The case could disrupt the Sarkozy's plan to stand in the Republicans' primary in November ahead of next year's presidential election.
"The same people who say that the authority of the state is being challenged ... are, by comments that damage the legal system, challenging the authority of the state," Valls commented.
Prosecutors say that Sarkozy was "the principal beneficiary" of false accounting that allowed his campaign to spend more than twice as much as allowed by law and that he ordered more expenditure while knowing that accountants had already warned that the legal limit had been exceeded.
Both Herzog and Kuster say that the overspending case has already been dealt with because the campaign's accounts were rejected for overspending by 466,118 euros by the Constitutional Council in 2013.
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