MILAN, July 21 (Reuters) - Banca Monte dei Paschi di Siena and its former top investor Fondazione Monte dei Paschi said on Wednesday they had reached a preliminary accord to settle their outstanding legal disputes.

The foundation last summer filed 3.8 billion euros ($4.5 billion) in extra-judicial claims against Banca Monte dei Paschi, linked to its acquisition of rival Banca Antonoveneta in 2008 and its capital increases in 2011, 2014 and 2015.

Italy's Treasury, which is now the bank's largest shareholder with a 64% stake, has been working to halve 10 billion euros in legal claims facing Monte dei Paschi, including those from the foundation.

"The preliminary agreement allows the bank to reduce claims by an amount worth 3.8 billion, giving a significant contribution to solving the main element of uncertainty weighing on the bank's balance sheet," the lender and the foundation said in a joint statement.

As part of the transaction Banca Monte dei Paschi di Siena will pay the foundation 150 million euros, they said. A final deal to end any outstanding litigation will be submitted for approval to the lender's board on Aug. 5. ($1 = 0.8480 euros) (Reporting by Giulio Piovaccari; editing by David Evans)