Uranium One Inc. said 80 percent of workers at its Dominion South Africa mine are on an illegal strike for a third day, halting production.

"We're doing everything in our power to get them back to work as soon as possible,'' Robert van Niekerk, executive vice president of technical services, said by phone today. Workers have ignored "several ultimatums,'' he said.

Former employees dismissed for misconduct "encouraged'' about 2,000 workers to strike on Oct. 7, Van Niekerk said. The disruption will hinder the company's efforts to regain investor confidence after missing production targets this year and last, and following an 85 percent slump in its shares so far in 2008.

Uranium One today tumbled 20 percent to a record low of 8.58 rand in Johannesburg trading, giving the company a market value of 4 billion rand ($439 million).

JPMorgan Chase & Co. cut its forecast for uranium prices through 2010 on increased spot-market sales of the radioactive metal in September and the potential for the credit freeze to slow development of nuclear power projects. Prices for immediate delivery may average $65.98 a pound this year, JPMorgan said in a report yesterday, from an earlier forecast of $69.62.

Uranium-oxide spot prices, which reached a record $138 a pound in June last year, dropped $4 to $49 in the week ended Oct. 6, according to The Ux Consulting Co. LLC's Web Site.

To contact the reporter on this story: Carli Lourens in Johannesburg at clourens@bloomberg.net