LONDON (AP) — NASCAR indefinitely suspended driver A.J. Allmendinger on Tuesday after confirming he violated its substance-abuse policy, putting his career and future with Penske Racing in jeopardy.

The suspension came after a test of his backup urine sample confirmed the original positive result.

Allmendinger originally was suspended July 7 after failing a random drug test taken in late June. His backup “B” urine sample was tested Tuesday by Aegis Analytical Laboratories in Nashville, Tenn.

NASCAR officials announced the result and subsequent suspension Tuesday night.

“While not a pleasant experience by any stretch of the imagination, we feel like we have one of the best drug testing programs in sports,” NASCAR spokesman David Higdon said.

Allmendinger on Twitter thanked fans for their support.

“I’m sorry we even have to have this going on. But I promise I will do whatever it takes to get to the bottom of this and get back out there no matter what,” he tweeted.

Officials did not announce what substance Allmendinger tested positive for. Allmendinger previously said he tested positive for a stimulant, but did not give specifics. He has denied knowingly taking a banned substance.

NASCAR officials did not say how far Allmendinger’s positive test went beyond a predetermined range of acceptable levels — and as is the case in other sports drug testing programs, officials wouldn’t consider it a valid defense if a competitor just barely exceeded acceptable levels of a banned substance.