Aon Inc. has agreed to purchase Gallagher Re in a deal worth $45 million.

The deal was announced less than a month after Arthur J. Gallagher Inc., Itasca, Ill., announced it was putting the company up for sale.

The transaction will involve an initial cash payment of $30 million from Aon, Chicago, and additional payments over the next 12 months. Gallagher said the payments should amount to an additional $15 million cash after closing.

The agreement involves Gallagher Re’s U.S. and U.K. reinsurance brokerage businesses, with about 50 employees going from Gallagher to Aon, according to someone familiar with the arrangements.

Michael D. O’Halleran, executive chairman of Aon Re Global, said the purchase would give Aon a larger presence in the U.S. accident, health and life markets and increase its capabilities in the U.K. specialty, casualty and financial institutions businesses.

In a separate statement, Gallagher said the U.S. portion of the deal was completed Friday and that it expects the U.K. purchase to be completed on or before April 30.