South Korea's anti-trust agency has fined drugmaker GlaxoSmithKline three billion won ($2.6 million) for conspiring with a Seoul rival over the sale of drugs, an official said Monday.

The (FTC) said GSK in 2000 offered Dong-A Pharmaceutical the exclusive right to sell its anti-nausea drug Zofran and an anti-viral medication called Valtrex.

In return, the FTC said the British-based multinational demanded that Dong-A stop selling a cheaper of Zofran which the Seoul firm had introduced in 1998.

GSK also asked that Dong-A never produce or sell any drugs that could compete against Zofran and Valtrex, Kim Jun-Ha, a FTC official involved in the case, told AFP.

"With the cheaper generics made by Dong-A taken off the market, the on patients and on the government's health insurance budget has increased," Kim said.

The FTC said in a statement it estimated GSK had made wrongful gains of about 16 billion won. It said "the two firms shared benefits that were to go to consumers".

Dong-A was fined 2.1 billion won for the collusion, the agency added.

GSK's South Korean unit called the decision "very regrettable" and "inappropriate" and said it would appeal in court.

"We simply exercised our legitimate patent rights," it said in a statement, adding that there was no illegality.