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).
BG Group’s shares opened up 42% on the London Stock Exchange at 1,293.5p.
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.