Two athletes who competed at this year's world championships in Paris tested positive for the designer steroid THG, the International Association of Athletics Federations (IAAF) says.

The ruling body ordered the Paris samples to be retested after the discovery of the previously undetectable substance.

IAAF, whose council started a two-day meeting in Berlin on Saturday, said there were two positive 'A' samples from the 402 further tests conducted. No names or nationalities were released.

The relevant federations have been notified and will now request an explanation from the athletes and, if necessary, proceed to the analysis of 'B' samples.

The athletes have up to 21 days to request such a test, and the 'B' test must be analysed within 28 days of that decision.

A source close to the Berlin meeting said the positive tests concerned athletes who had either already been tested positive or belonged to a group linked with the BALCO lab.

The U.S. Anti-Doping agency said BALCO, a firm located south of San Francisco which has many top sportsmen and women among its clients, was probably the source of THG.

Several leading track athletes and baseball players, among them triple Olympic sprint champion Marion Jones and her partner Tim Montgomery, the 100 metres world record holder, have already testified before a grand jury investigating BALCO. More will follow.

Five athletes, including European 100 metres champion Dwain Chambers of Britain, have tested positive for THG.

The other four have not been officially confirmed but the growing scandal clearly threatens the credibility of the showcase Olympic sport.

Possible anti-doping rule changes will be discussed in Berlin. Some member federations believe the IAAF should reintroduce harsher penalties and revert to a minimum four-year ban for a major doping offence instead of the two-year suspension which the ruling body moved back to in 1997.