Centre-left leader Pier Luigi Bersani labelled it a “scam”, while the leader of a smaller party accused Silvio Berlusconi of committing a crime.
The letter was sent to voters in swing regions of Italy.
It vows to scrap the unpopular property tax brought in under ex-Prime Minister Mario Monti and pay voters back.
Silvio Berlusconi had already made the pledge on the campaign trail, but putting it in a letter to voters outraged his opponents.
“If I’d crossed paths with Berlusconi after the letter was sent, I’d have told him he’s a cheat,” said Pier Luigi Bersani, whose long-time lead in the polls has been gradually cut back by Silvio Berlusconi.
Former anti-mafia prosecutor Antonio Ingroia, who leads new left-wing Civic Revolution party, said on his website: “With the letter sent to Italians promising money in exchange for votes, Berlusconi has committed one crime, possibly two.”
He called for Silvio Berlusconi to be prosecuted.
The letter came in an official-looking envelope, headed: “Important notice: reimbursement of IMU 2012.”
“The refund will be available either through a transfer into your bank account, or to you personally at the counter of the post office,” the letter said, according to Reuters.
It was sent to millions of households in Sicily, Veneto, Campania and Lombardy – key regions which could decide the result of the election, which is held on Sunday and Monday.
The IMU tax amounts to 0.4% of the value of a property owner’s primary residence, and is comparable to similar taxes levied elsewhere in Europe.
Polls have shown it is the most unpopular of the budget measures brought in by the Monti government to try to restore confidence in Italy’s finances.
Market confidence in Italy collapsed during 2011 because of its very high debt burden, and only began to recover after Mr Berlusconi resigned as prime minister and Mr Monti took over.
Silvio Berlusconi has declared: “The house is sacred. It should not be taxed.”
But Mario Monti, who is also heading an election coalition, retorted: “When I said that Berlusconi would try to buy the votes of the Italian people with the money of the state, I didn’t think he would [do it] to the letter.”
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