Nomura Holdings cemented its position at the top of the M&A league tables in Japan by bagging a mandate to advise Dainippon Sumitomo Pharma on its $2.6 billion acquisition of U.S. drugmaker Sepracor Inc.

U.S. investment bank JP Morgan also came out as one of the winners as it landed an advisory role to Sepracor, making it an adviser for Japan's two biggest deals so far this year.

Thomas Weisel Partners LLC, a Nomura partner in M&A deals, is also advising Dainippon Sumitomo, while Jeffries & Co is working with JP Morgan in advising Sepracor.

Dainippon, Japan's No.7 drugmaker by sales, said on Thursday it would buy Sepracor in Japan's second largest overseas acquisition this year, after Kirin Holdings' buyout of Australian brewer Lion Nathan Ltd.

The deal should provide a boost for Nomura and JP Morgan which have faced falling M&A volumes in the uncertain economic climate.

Japanese firms have spent $41 billion on acquisitions so far this year slowing from $64.5 billion a year ago, according to Thomson Reuters.

It is positive news for Nomura that it has retained this role at a time when the deal flow is slowing down, said Azuma Ohno, a brokerage analyst at Credit Suisse Securities.

Nomura probably used its relationship with Sumitomo Chemical Co, which owns 50.1 percent in Dainippon Sumitomo, to secure the latest mandate, some analysts said.

Nomura is listed as the top relationship broker for Sumitomo Chemical in a corporate guide book called Shikiho published by Toyo Keizai.

Nomura was the top adviser for Japanese companies' mergers and acquisitions for the six months to June this year, according to Thomson Reuters data.

Last year, Nomura advised Japanese drugmaker Daiichi Sankyo in its purchase of a majority stake in Indian generic drugmaker Ranbaxy Laboratories for around $5 billion.

That deal proved to be a headache for Daiichi, which was forced to write down its stake following a sharp slide in Ranbaxy's stock, pushing it to a loss of more than $3 billion in the past business year. (Editing by Anshuman Daga)