Shares in platinum producer Lonmin surged 48.80 percent or 162 rand to trade at 494 rand on the JSE by 10.50am on Wednesday.

Lonmin, which has operations in South Africa, on Wednesday called the US$10-billion offer for its company by Xstrata "opportunistic and entirely unwelcome".

Xstrata announced its plans to bid for Lonmin on Wednesday morning, saying the proposed offer represents a cash premium of 42 percent to Lonmin's share price of £23.19 at the close of business on Tuesday.

Lonmin said in a statement that it had noted the unsolicited, pre- conditional offer for its entire issued share capital.

"This is an opportunistic and entirely unwelcome attempt to acquire Lonmin at a price which undervalues its unique assets," said Lonmin.

The company said a further announcement would be made by the board shortly.

"In the meantime, shareholders are strongly advised to take no action in respect of the pre-conditional offer," Lonmin said.

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