* Berlusconi now five points behind centre left
* Centre left loses support over bank scandal
* Pollsters still think Berlusconi unlikely to win
By Barry Moody
ROME, Feb 1 (Reuters) - A growing scandal at Monte dei Paschi di Siena, the world's oldest bank, has helped Silvio Berlusconi close the gap with the centre-left frontrunners to five percent, its tightest so far, weeks before Italy's election, a poll showed on Friday.
An SWG survey showed former premier Berlusconi's centre-right coalition had gained 1.3 percent in a week and was now on 27.8 percent, compared to 32.8 percent for Pier Luigi Bersani's centre left, which had lost 1.6 percent.
Berlusconi, a 76-year-old billionaire media magnate, has made an astonishing comeback over the last month through a blitz of radio and television appearances, but SWG said the Monte dei Paschi scandal had also had a significant impact.
Bersani's Democratic Party (PD) has ties going back decades with the 540-year-old Tuscan bank through local government and dominance of a charitable foundation which is the lender's biggest shareholder.
SWG said the scandal had both galvanised previously disillusioned centre-right voters and demoralised centre-left supporters ahead of the Feb. 24-25 vote.
Some 50 percent of those polled said the PD had responsibility for the trouble at Italy's third largest bank.
Bersani says the national party was not to blame for a scandal over murky derivatives contracts and cash problems that have forced Monte Paschi to take a state bailout.
Outgoing Prime Minister Mario Monti's centrist grouping has also gained over the last week by 1.4 points, but he is stuck at 14.2 percent, far behind his 20 percent target and well beaten into third place by the surging anti-establishment 5-Star Movement.
The movement, led by Genoese comic Beppe Grillo, has gained just under 1 percent and is on 18 percent, its best result for weeks. It has exploited not only a series of political scandals but deep voter anger over the pain of Italy's deep recession.
Monti has disappointed hopes among investors and foreign leaders that he could sweep away the old, discredited political order and lead a fundamental renewal of Italy.
Most pollsters still think it unlikely that Berlusconi can overtake Bersani with little more than three weeks to go, but before the media magnate entered the race in December after months of indecision, his PDL part looked close to collapse and was polling around 15 percent. Now the party alone is polling 19.3 percent in the SWG survey.
Because of a much maligned electoral law, Bersani only needs to be ahead to take a winner's premium that will give him comfortable control of the lower house of parliament.
But the race will be decided in the Senate, which has equal law-making powers and where bonus seats awarded to the winner are allocated on a regional basis.
Berlusconi's aim is to win enough senators to stymie a centre-left government, although ironically his advances could push Bersani into a post-election coalition with Monti.
Pollsters say the Senate election will be decided in three or four battleground regions where the result is too close to predict, especially Italy's most populous region Lombardy, which returns the most senators. These results may not reflect the nationwide poll ratings.
Despite Monti's failure to gain traction, pollsters think Bersani will fall short of control of the Senate and will have to invite the former European commissioner to join a coalition, although the latter's power of negotiation will depend on how many votes he wins.
Berlusconi was driven out of power in November 2011 when Italy's borrowing costs soared and it slid towards a Greek-style crisis. He was replaced by Monti who brought Italy back from the brink and restored its international reputation.
After emerging from months in the shadows in December, Berlusconi, a born showman, has steadily clawed his way back in the ratings, attacking Monti over a hated property tax and accusing him of being a German puppet.