MILAN (Reuters) - Troubled Italian bank Monte dei Paschi di Siena <BMPS.MI> said on Wednesday it had sold its stake in medical technology group Sorin <SORN.MI>, as it continued planned asset sales to help return to profit and avoid nationalisation.

The world's oldest bank, reeling from derivatives losses and weakened by years of economic crisis, said it sold its entire stake of around 5.7 percent in Sorin for 56.3 million euros (46.52 million pounds), through an accelerated book-building process.

The Tuscan bank is searching for every available source of revenue after shareholder opposition forced it to delay a 3 billion euro share sale which it needs to raise capital to pay back billions of euros in state aid.

The capital hike, originally planned for January, has now been delayed until the middle of the year.

If the bank cannot repay the aid by the end of the year, it faces the prospect of nationalisation.

Italy's market watchdog has asked Monte Paschi if it will challenge in court the decision by the main shareholder foundation to delay the rights issue, a source familiar with the matter said earlier on Wednesday.

Equita Sim acted as bookrunner for the sale of the stake in Sorin.

($1 = 0.7353 euros)

(Reporting by Isla Binnie, editing by David Evans)