SIENA (Reuters) - The top investor in Italy's Monte dei Paschi dei Siena <BMPS.MI> is open to the idea of a merger of the bank with another financial institution, preferably "of an international standing", it said in a document approved by its board and seen by Reuters.

The bank, the world's oldest, risks nationalisation unless it can raise 2.5 billion euros (2.1 billion pounds) in a cash call to be carried out next year.

The Monte dei Paschi foundation, a not-for-profit entity with close ties to local politicians, has a 33.5 percent stake in the bank but is seeking to reduce that to pay back 350 million euros of debts.

In the document, dated October 15 and setting out its strategic priorities through 2017, the foundation said it did not want to sell its stake "in dribs and drabs".

(Reporting by Silvia Ognibene, writing by Silvia Aloisi)