A spike in the share price of BHP target Anglo American's London-listed shares before the Australian multinational had formally announced a $60 billion takeover bid has raised questions about possible leaks.

Shares in Anglo American rose by around 2.8 per cent between 3pm GMT (12am AEST) and the market close at 3.30pm GMT in London on Wednesday.

It issued a statement nearly eight hours later, at around 11pm GMT, saying it had received an approach from BHP.

The late bounce before the bid proposal was revealed, which if successful would forge the world's biggest copper miner, contrasted with modest share price gains of between 0.2-0.5 per cent at Anglo's peers Rio Tinto, Glencore and Antofagasta in the 35 minutes before the London market closed.

"This spike in Anglo's share price before the public announcement of the BHP bid raises questions about the integrity of the UK's markets which the Financial Conduct Authority (FCA) are likely to want to investigate," said Harvey Knight, head of the financial services regulatory group at law firm Withers.

Britain's FCA does not typically comment on individual trading irregularities which it investigates and declined to comment on whether it would review the activity.

In a statement emailed to Reuters, the regulator said it "cannot comment on individual cases".

A spokesperson for Anglo American declined to comment when asked whether the company had contacted the FCA about any potential irregular trading or whether the regulator had been in touch.

Shares in miners like Anglo American can move sharply following changes in commodity prices, or major currencies like sterling and the US dollar.

Regulated companies and individuals in Britain have an obligation to detect and report suspicious trading if there are reasonable grounds to suspect market abuse, such as insider dealing or market manipulation.

"Between private chat networks, secure texting apps, and even burner phones, there are many more ways for material, non-public information to be shared via undetected means than there ever were previously," Peter Earle, senior economist at the The American Institute for Economic Research, told Reuters.

"The financial markets are in an information dissemination arms race. Some of it is above board, and some of it below."

BHP said it will offer 25.08 British pounds (A$48.05) per Anglo American share, a premium of 31 per cent from Wednesday's close, and spin out its London-listed target's iron ore and platinum assets in South Africa, where BHP, the world's largest listed miner, has no activities.

Anglo American, which owns mines in countries including Chile, South Africa, Brazil and Australia, said it was reviewing the unsolicited and non-binding proposal from BHP, which under UK takeover rules has until May 22 to make a firm offer.

If the deal goes through, it would give BHP access to more copper, one of the most sought-after metals for the clean energy transition, and potash, which are its key strategic commodities, as well as more coking coal in Australia.

Two sources told Reuters Anglo American does not consider the proposed offer as attractive, with one saying it did not address the complexities of demerging the Anglo American Platinum and Kumba Iron Ore businesses in South Africa.

Another source said the Anglo American board is meeting on Friday London time to finalise their thinking on the proposal.

Several Anglo American shareholders voiced concern over BHP's move, with Legal & General Investment Management calling it "highly opportunistic" and "unattractive".

Redwheel, meanwhile, said details were "rather sketchy", adding that BHP's offer could spur further interest in Anglo American.

South Africa's government is also scrutinising the takeover bid, a spokesperson said, with the deal likely to concern officials in the coming weeks before a general election where the governing party's majority is at risk.

The deal, which would involve Anglo American exiting its platinum and iron ore assets in South Africa, could trigger large capital outflows and further dent the country's reputation as a destination for mining investment.

The potential exit would mark the end of an era for a company founded in Johannesburg over a century ago.

Reuters