Mining heavyweights Glencore and Xstrata have unveiled a deal to create the world's fourth largest natural resources firm worth 90 billion US dollars (£57bn).
The pair - among the top 20 firms on the London stock market - will have operations in 33 countries and should be better able to compete against bigger rivals BHP Billiton and Rio Tinto if the merger is backed by shareholders.
Glencore, the world's biggest commodities trader, with products including oil, coal, gold and foodstuffs, has been circling Xstrata for a number of years and has already built up a 34% stake in the company.
Last year, Glencore became the first company in 25 years to be fast-tracked into the FTSE 100 Index in London's largest ever flotation.
Anglo-Swiss firm Xstrata is the world's largest exporter of thermal coal and also produces copper, nickel and zinc. Its shareholders will have a 45% stake in the proposed new firm.
Glencore chief executive Ivan Glasenberg said: "We have a fantastic opportunity to create a new powerhouse in the global commodities industry."
Glasenberg will be deputy chief executive in the new firm, with the top job going to current Xstrata boss Mick Davis.
The planned deal values Xstrata at 39.1 billion US dollars (£24.8 billion) and represents a 15.2% premium on its share price on February 1, the day before the pair confirmed talks over a potential tie-up.
It was not long before the announced merger sparked concern with one major institutional investor - Edinburgh-based Standard Life Investments, which holds around 63.3 million shares in Xstrata, or 2.15% of the total.
The company said it would vote against the merger plans unless terms of the deal are improved, as it currently undervalues Xstrata's assets.
David Cumming, head of equities at Standard Life Investments, said: "Although we see some merit in the merger of Xstrata and Glencore, the proposed exchange ratio clearly undervalues Xstrata's assets and future earnings contribution.
"Consequently it is our intention to vote against the deal unless the merger terms for Xstrata shareholders are materially improved."
The move echoes concerns of shareholders in security group G4S, which last year resisted a proposed multibillion-pound takeover of a Danish cleaning firm.
G4S scrapped the move - just two weeks after it was announced - after shareholders raised concerns over the scale of the takeover amid an uncertain economic climate.