Silvio Berlusconi has caused outrage among political rivals by sending letters to millions of Italians promising to reimburse tax they paid in 2012, in a final drive for votes before elections this weekend.
The official-looking envelopes read: "Important notice: reimbursement of IMU 2012", referring to an unpopular property tax brought in by the technocrat government of Mario Monti.
"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 letters say.
Democratic Party leader Pier Luigi Bersani, whose centre-left coalition was leading in the last opinion polls published before a pre-election blackout, called the letter a "scam".
Former anti-mafia prosecutor Antonio Ingroia, who leads a small centre-left group, called for Mr Berlusconi to be prosecuted, claiming the letter promised money in exchange for votes which was a crime.
Mr Berlusconi has made strong gains in polls since the start of the year in his campaign to return to power. He made the "shock proposal" to repay the levy in early February and has repeated it at every opportunity, hoping for a repeat of the lift he won with a similar pledge before the 2008 election.
Households in swing regions Sicily, Veneto, Campania and Lombardy, which could decide the outcome of the race for the Senate, received extended versions of the letter, according to the daily La Repubblica.
Surveys have shown IMU to be one of the most unpopular measures introduced by Mr Monti's government, Reuters reports.
According to a poll by the Demopolis institute in early February, 51% of Italians do not believe Mr Berlusconi's pledge, and only 15% think it is credible.