MILAN (Reuters) - Ratings agency DBRS has placed the credit rating of Banca Monte dei Paschi di Siena under review saying a new restructuring plan for the world's oldest bank could disrupt some areas of its business.

The agency said it would review the Italian bank's current long-term debt and deposit rating of BBB and short-term rating of R-2 (middle) for the bailed-out bank, with "negative implications".

Monte Paschi, under investigation over the costly acquisition of a rival in 2008 and its use of loss-making financial derivative instruments, has been told by the European Commission to raise 2.5 billion euros ($3.42 billion) via a capital increase more quickly than previously planned.

"Having a shorter time frame makes it harder for the bank to demonstrate progress towards achieving its strategic targets," DBRS said in a statement.

Italy's third-largest bank presented a restructuring plan on October 7, including thousands of job cuts. The planned capital hike is roughly equivalent to its market value.

DBRS said the plan to complete the capital raising by the end of 2014 may coincide with market uncertainty associated with European Central Bank stress tests on euro zone banks and raise the risk of state-backed debt being converted into government shares.

"Such a conversion would give the Italian authorities a controlling stake in Monte dei Paschi and could add to uncertainty regarding the bank's future direction," DBRS said.

If the bank meets the European Commission's strict timeline and cuts costs, improves revenue generation and reduces its reliance on ECB funding, the current ratings could be confirmed, DBRS said.

However, any delay or evidence of weakness in Monte dei Paschi's third-quarter results, due on November 14, could have negative implications for the current ratings, the agency added. ($1=0.7319 euros)

(Reporting by Isla Binnie; Editing by Greg Mahlich)