NASCAR Sprint Cup driver AJ Allmendinger has been temporarily suspended from competition, after he was reported to have failed a random drugs test that was carried out last weekend at Kentucky Speedway.

The official notice from NASCAR stated that Allmendinger's 'A' sample had tested positive for a banned substance and that he was suspended from NASCAR competition with immediate effect.

Steve O'Donnell, NASCAR's senior vice president for racing operations, made the press announcement to the media in Daytona but would not be drawn on elaborating or giving any further details.

"NASCAR will follow its policies and procedures set forth in the rule book in dealing with this matter," said O'Donnell.

It's not known what substance the 'A' sample has reported positive for. Allmendinger was previously arrested by police for drunk driving in October 2009, but he did not miss any NASCAR races over the charge. However, commentators have pointed out that this latest test would not normally be used to identify alcohol levels and that it would therefore likely be a different type of substance that had been detected. NASCAR has a long list of prohibited substances for competitors, including some otherwise legal prescription medications.

Allmendinger can request in the next 72 hours that the 'B' sample drawn at the same time as the 'A' sample is tested, in order to ensure that the first sample has not become contaminated. If that sample also tests positive, or if Allmendinger does not request that the 'B' sample be tested, that Allmendinger's suspension would become indefinite under NASCAR's Substance Abuse Policy.

In that event, and if the alleged presence of drugs was thereby confirmed, Allmendinger would need to undergo a mandatory rehabilitation program before he would be allowed to compete in NASCAR again, a process that could take several months.

Allmendinger had been scheduled to drive in Saturday night's Coke Zero 400 race at Daytona International Speedway in the Penske Racing #22 car and had qualified in ninth place on the grid. As a result of the suspension, the team had to urgently fly in Sam Hornish Jr. to take over the car for the race.

Hornish had already returned home to his home in North Carolina after competing in last night's Nationwide Series race for the team, and was undertaking media interviews when the urgent call came through for him to get back to Daytona as soon as possible. He arrived just minutes before the cars started rolling off pit road.

Penske Racing was not taking calls about the suspension, but did release a team statement in which they said: "NASCAR notified Penske Racing this afternoon that AJ Allmendinger was administered a drug test earlier this week, and those results tested positive.

"NASCAR has a strict drug-testing program that Penske Racing fully supports. Penske Racing will work with NASCAR through this process and its next steps. Sam Hornish Jr., will drive the #22 car in tonight's Coke Zero 400."

Allmendinger's Cup team mate at Penske, Brad Keselowski, also made clear that he would not be answering any questions on the matter: "No comments on 22 car tonight as we focus on tonight's race. Thank[s] in advance," he tweeted.

There was no statement from Allmendinger himself.

Other than organising an emergency replacement driver for the weekend, the team have not been able to have internal discussions about the crisis since it broke earlier today. Roger Penske is still in the process of returning from holiday in Europe and team president Tim Cindric is in Toronto overseeing the organisation's IndyCar weekend race operations.

Allmendinger joined Penske from Richard Petty Motorsports at the start of the 2012 season following the team's decision to part company with former driver Kurt Busch.

The last high-profile driver to fall foul of NASCAR's random drug testing policy was Jeremy Mayfield in 2009. The legal fall-out from that case - with Mayfield counter-suing NASCAR over his suspension - has only recently come to a conclusion.