Royal Dutch Shell says it has agreed to buy oil and gas exploration firm BG Group in a deal that values the business at £47bn.
The two firms say they have reached agreement on a cash and shares offer which gives investors a 50% premium on BG Group's share price on 7 April.
The deal could be one of the biggest of 2015 and could produce a company with a value of more than £200bn ($296bn).
Shell's £177bn market capitalisation dwarfs that of BG, which now stands at £31bn after a 20% fall in its share price over the past year.
Dividends
BG Group is the UK's third largest energy company, and currently employs about 5,200 people in 24 countries.
It was created in 1997 when British Gas demerged into two separate companies: BG and Centrica.
BG took control of exploration and production while Centrica took charge of the UK retail business of the former British Gas.