Brazilian 800m runner Fabiane dos Santos is out of the World Championships after testing positive for testosterone.

The IAAF confirmed that the Brazilian Athletics Federation had suspended Dos Santos after she tested positive for testosterone at the Brazilian Grand Prix in Rio de Janeiro on 6 May this year.

Dos Santos burst onto the international scene this summer with a series of startling performances.

She ran Olympic champion Maria Mutola close at the Golden League meeting in Monaco and then won the British Grand Prix at Crystal Palace, out-sprinting Olympic silver medallist Stephanie Graf.

Britian's Kelly Holmes spoke last week of her surprise at Dos Santos' incredible rate of improvement over the past 12 months.

"Dos Santos was ranked 76th last year so she has made good progress," said Holmes.

"She barges her way through, and that shows her inexperience of racing. She is very strong."

Canadian sprinter Venolyn Clarke became the first athlete to fail a drugs test at the Championships.

The 100m runner was tested during the Canadian team's pre-championships camp in Calgary.

It was announced on Tuesday that her sample contained traces of the banned anabolic steroid stanozolol.