Halkia was the surprise winner in Athens four years ago ahead of injured Australian Jana Rawlinson, who finished fifth.

Rawlinson, who had been pre-race favourite in Athens and Beijing, has withdrawn from these Games with another injury.

It leaves the field for the women's 400m hurdles wide open. Halkia was summoned on Saturday night by the head of Greece's Olympic delegation and told of the results of the first sample she gave to World Anti-Doping Agency doctors.

''I am shocked,'' she said yesterday morning. ''I have undergone more testing than anyone else.''

Halkia, 29, was tested a few days before the Beijing Olympics in Japan, where Greece's track and field team had been training.

She said she had volunteered to take part in the agency's pilot program in which athletes submitted themselves voluntarily to regular testing.

Halkia, who has moved out of the Olympic village, said she was sorry she could not take part in the Games and that she had expected to make the 400m hurdles final.

The 400m hurdles first-round heats were being run last night.

Halkia said she did not know how the banned substance was found in her sample.

Fifteen Greek athletes, including Halkia, have tested positive for methyltrienolone. They include 11 weight-lifters, swimmer Yannis Drymonakos and sprinter Tassos Gousis, who was sent home a few days before the Olympics.

The International Olympic Committee has also barred sprinter Katerina Thanou, from Beijing, for her role in a drug-testing scandal at the Athens Games.

Halkia was a relative unknown before she won the gold medal in 2004.

Her semi-final time of 52.77sec set an Olympic record. AAP/AP