Anglo American has rejected rival BHP's $60 billion takeover proposal, saying the bid significantly undervalued the London-listed miner and its prospects.

In a statement, Anglo American said it had unanimously rejected the proposal, and that it believed the offer was unattractive for shareholders.

Anglo American chairman Stuart Chambers said the proposal was "opportunistic".

"The proposed structure is also highly unattractive, creating substantial uncertainty and execution risk borne almost entirely by Anglo American, its shareholders and its other stakeholders.

"Anglo American has defined clear strategic priorities — of operational excellence, portfolio, and growth — to deliver full value potential and is entirely focused on that delivery."

Earlier on Friday, questions were raised about a potential leak over a spike in the share price of Anglo American's London-listed shares before BHP formally announced its takeover bid.

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.

Reuters