* Monti says tax-cut promises must be credible
* Bersani's bloc at 38-39 pct, Monti's at 14-15 pct - poll
* Berlusconi's bloc at 26-28 pct, 5-Star Movement at 13-14 pct
Italy's outgoing prime minister, Mario Monti, said on Sunday he would alter an unpopular property tax imposed by his own government, as a poll showed his centrist bloc gaining in popularity ahead of next month's election.
Monti's new centrist formation was third in a survey published on Sunday ahead of the Feb. 24-25 parliamentary vote, behind a centre-left coalition led by Pier Luigi Bersani and the centre-right bloc of former prime minister Silvio Berlusconi.
Monti, a 69-year-old former European commissioner, was appointed in November 2011 to lead an unelected right-left government of experts after Berlusconi quit amid a sex scandal and Italy's financial crisis threatened the euro.
To cut the deficit and restore confidence in Italy's ability to manage its 2-trillion-euro debt, Monti introduced a series of austerity measures in late 2011, including a property tax that hurt consumer spending and deepened the recession.
Berlusconi, who supported Monti's government until two weeks ago, has repeatedly said his first act of government, should he win February's vote, would be to abolish the property tax.
"Taxes need to be cut, but no one should be making promises that cannot be kept," Monti told SkyTG24 television. The property tax "should be restructured and modified", he said, with a greater portion set aside for city governments.
Monti repeated he wants to cut income taxes for low earners and said a planned value-added tax increase can be averted if the election winners are "ready to say no to special interests".
Both Berlusconi and Monti have made multiple appearances on TV, in Twitter question-and-answer sessions, and in online video interviews over the past week as they seek to close the gap with the centre-left, and it is paying off.
The centre-left coalition still has a comfortable lead, but both Monti's and Berlusconi's blocs have gained in recent weeks, according to a poll published on Sunday.
The number of voters who say they will vote for Bersani's bloc is between 38 and 39 percent, and the Democratic Party (PD) is seen getting 32-33 percent, polling institute ISPO said.
Monti's bloc has risen to between 14 and 15 percent from just over 10 percent before he entered the race, and Berlusconi has boosted his own party's share to 17-19 percent from 13-16 percent at the beginning of December, the poll indicated.
The number of undecided voters, or those who plan to abstain, has fallen below 40 percent, down from almost 50 percent a few weeks ago, the poll showed.
The new "With Monti for Italy" formation presented on Friday would itself win 9 percent, the poll said, and is drawing votes mostly from the centre-left and the previously undecided, said Renato Mannheimer, head of ISPO.
"Most analysts see it as improbable that, as things now stand, the coalition led by Monti can win more than 20 percent," he said.
If Berlusconi seals an alliance with the regionalist Northern League, his coalition could pull in 28 percent of the vote, ISPO said. Most of the PDL increase came from the large pool of undecided and disillusioned voters, Mannheimer said.
If Berlusconi and the League run together, complexities of the electoral law might make a post-election alliance with Monti key to a stable Senate majority for Bersani, Mannheimer added.
The anti-establishment 5-Star Movement, led by comic Beppe Grillo, dropped to 13-14 percent from 17-19 percent a month ago.