Mitsui Sumitomo Insurance Co. said Tuesday that it will buy Britain's nonlife insurer Amlin PLC for around ¥635 billion, as it pursues global expansion in the face of a shrinking domestic market.

The deal will make Amlin a wholly owned subsidiary of Mitsui Sumitomo Insurance, a unit of MS&AD Insurance Group Holdings Inc. It is expected to be complete by the end of March 2016, the Japanese insurer said.

Operating in the Lloyd's reinsurance market, Amlin sells marine and aviation insurance as well as reinsurance, a type of insurance bought by insurance companies.