ROME (Reuters) - Italy's Banca Monte dei Paschi di Siena will seek 2.5 billion euros ($3.3 billion) in extra capital from investors, more than double its original plan, under a revised bailout to shore up the loss-making bank, the economy ministry said on Sunday.

The revised plan is the latest measure in a painful recovery process for Italy's number three bank, still grappling with the aftermath of a massive derivatives scandal which emerged in the wake of its expensive acquisition of rival Antonveneta in 2008.

The government has already offered 4.1 billion euros of state loans and the recapitalisation, to take place next year, would more than double an originally planned 1 billion euro capital hike.

It would also match the current market capitalisation of Monte Paschi, which stood at 2.5 billion euros at market close on Friday, according to data from Thomson Reuters.

The statement from the Treasury followed a meeting between Economy Minister Fabrizio Saccomanni and EU Competition Commissioner Joaquin Almunia at the sidelines of a conference at Cernobbio on the shores of Lake Como on Saturday.

Almunia also made clear after the meeting that if Monte Paschi failed to find enough private investors for its capital increase, the Italian state, which has so far avoided nationalising any of its weaker banks, would have to step in and convert state aid into bank shares.

The plan will include new cost cuts and a gradual reduction in the bank's huge government bond portfolio which totalled 29 billion euros at the end of June although the ministry said it would not affect the bank's role as a market operator.

The plan will be considered by the bank's board and approved by the government and the Bank of Italy before being submitted to European Union authorities for clearance under state aid rules. The ministry said it expected the approvals process could be completed within two months.

The recapitalisation will aim to repay a significant portion of special bonds bought by the Italian Treasury ahead of the schedule in the current bailout plan, the ministry said.

No comment was immediately available from the bank.

($1 = 0.7600 euros)

(Reporting by James Mackenzie and Lisa Jucca; editing by Patrick Graham)