The merger offer would create the world's biggest gold company

The move by Harmony is said to have the backing of a number of shareholders, including Russia's Norilsk Nickel.

They are thought to disagree with a plan to join up with Canada's Iamgold.

Harmony said the offer valued Gold Fields at 52.9bn rand ($8.24bn). The new merged firm would have a combined market value of $10bn (�5.5bn).

It has offered 1.275 shares in the new company for each Gold Fields equity.

Putin worries

Norilsk currently owns a 20% stake in Gold Fields, which is the world's fourth largest producer.

However, Gold Fields' plans to merge with Iamgold are said to have upset Norilsk, which maintained it had been left in the dark over the proposal, the Financial Times (FT) reported.

What's the thinking behind Norilsk Nickel's strategy?

The paper claims that Norilsk has not backed the deal and recently stopped returning calls from Gold Fields executives.

Vladimir Potanin, the head of Norilsk, is one of Russia's biggest tycoons and is seen by analysts as a possible target for President Vladimir Putin in his clampdown on the country's business elite.

A merger with Gold Fields would enable some of Norilsk's resources to be moved out of Mr Putin's grasp, the FT said.

Should the merger is successful, it will be the latest example of consolidation in the gold sector.

Last year, AngloGold merged with Ghana's Ashanti to create AngloGold Ashanti, presently the world's largest gold company.