British Utility to Purchase KeySpan

British utility National Grid has agreed to buy New York-based electricity and natural gas distributor KeySpan Corp. for $7.3 billion in cash in a deal that would create the third-largest energy delivery utility in the United States.

National Grid said Monday that it also would assume debt estimated at $4.5 billion. KeySpan shareholders will receive $42 in cash per share.

Upon completion of the deal, National Grid will serve nearly 8 million customers in New York state and New England, the companies said.

“It’s more than simply getting bigger,” said Michael E. Jesanis, president and chief executive of National Grid USA. “For our investors, it’s also a win-win.”

National Grid, which owns and operates Britain’s power grid, already has a large presence in the United States.

It distributes electricity and natural gas to nearly 4 million customers in Massachusetts, New Hampshire, New York and Rhode Island.

KeySpan, based in Brooklyn, is the fifth-largest gas-distribution company in the U.S. and the largest in the Northeast. It operates the power transmission system for the Long Island Power Authority, conveying electricity to more than 1 million customers in New York’s Nassau and Suffolk counties and the Rockaway Peninsula in Queens.

KeySpan shares fell 65 cents to $40.76.

The acquisition is to be completed by early next year. National Grid said it expected $200 million a year in savings.

The British company said that it would honor all labor agreements in place between unions and KeySpan and that an undisclosed number of job cuts would be achieved through voluntary programs and attrition.