By Silvia Aloisi

SIENA, Italy, Dec 28 (Reuters) - The top investor in Banca Monte dei Paschi di Siena looked set to force a delay in a 3-billion-euro ($4 billion) capital increase the Italian lender needs to carry out to reimburse state aid and avert nationalisation.

The management of the world's oldest bank, led by Chairman Alessandro Profumo and CEO Fabrizio Viola, wants to launch the rights issue as early as January and has asked shareholders to approve it this week.

But the top shareholder - a not-for-profit foundation with close ties to Siena politicians - is determined to delay the cash call until May or later to win more time to sell down its stake in the bank and repay debt.

The cash call, along with a tough restructuring plan, is among the conditions imposed by the European Commission for approving a 4.1 billion euro state bailout that Monte dei Paschi received earlier this year.

A shareholder meeting got under way on Saturday after it was rescheduled because not enough investors showed up on Friday.

Thanks to its 33.5 percent holding, big enough to veto any decision, the foundation can force the bank to postpone the rights issue.

Tensions ran high ahead of the meeting, with sources close to the matter saying Profumo could quit if the capital increase is postponed.

The size of the capital increase is higher than the lender's stock market value of just over 2 billion euros and the operation is regarded as risky as the bank was hit hard by the economic crisis and a derivatives scandal.

Saddled with around 340 million euros of debt, the foundation is looking for a buyer for all or part of its Monte Paschi stake to pay back creditors.

It fears that a cash call next month would massively dilute its holding and leave it with virtually nothing to sell.

The bank's management wants instead to tap investors for cash as soon as possible and has secured a pool of banks to guarantee the share issue if it is launched before end-January.

Profumo said last week that a postponement would cause great uncertainty and could force the bank to be nationalised.

One specific concern is that Europe-wide health checks next year could force several other European lenders to raise capital, meaning Monte dei Paschi could find itself in a crowded market if it waits too long.

Under the agreement with the European Commission, if the Tuscan lender cannot complete the capital increase by the end of 2014 it will have to convert state loans it received in the bailout into shares issued to the Italian treasury.

The bank, which is cutting 8,000 jobs and shutting 550 branches as part of its turnaround plan, is hoping instead to pay back the bulk of the state aid through the cash call. It said a delay would cost it at least 120 million euros in interest payments.