Encke, winner of last season's Doncaster St Leger, was the highest-profile name among seven further Mahmood Al Zarooni-trained horses to test positive for anabolic steroids.

After the former Godolphin trainer's eight-year ban for administering banned drugs to 15 horses in his care, the British Horseracing Authority tested every animal at his Newmarket stable.

Those checks produced positive tests for another seven horses, including the Classic winner, with the steroid stanozolol found in their blood.

Testing at the other Godolphin stable of Saeed bin Suroor returned no positive results.

Encke will now be suspended from running for six months, after which just a few weeks of the Flat season remain.

The other six horses – Energizer, Genius Beast, Improvisation, Stamford, Steeler and Zip Top – will also be barred from racing.

Encke made great strides in 2012, graduating from a Sandown handicap via Group-race places at Goodwood and York to foil the Triple Crown bid of Camelot.

In beating Aidan O'Brien's colt, Encke ended Camelot's chances of emulating Nijinsky, winner of the 2,000 Guineas, Derby and Leger in 1970.

The BHA said Encke was tested after finishing third in the Great Voltigeur at York and after his Doncaster triumph, with the colt returning negative results on both occasions.

Given his negative post-race test in September, the BHA said it was satisfied Encke was clear of any banned substances when landing his Classic success and would not be disqualified.

Godolphin founder Sheikh Mohammed closed Al Zarooni's stable after the initial positive tests, and racing manager Simon Crisford said: "It is obviously very disappointing that seven further horses have tested positive.

"These results highlight why Sheikh Mohammed took the decision to lock down the stables."