Four-times Tour of Spain winner Roberto Heras on Tuesday attributed his positive doping test to a laboratory error.

Heras, 31, a former teammate of seven-times Tour de France winner Lance Armstrong, has been suspended after giving a positive test for the banned blood-boosting substance erythropoietin (EPO) during this year's race.

"They told me the news at the end of October. The only thing I can think of is that it is a laboratory error," Heras, one of the best climbers in the world, told radio station Onda Cero on Tuesday.

"My lawyers and the team are working on the case as we speak and there will be a 'B' test on Nov. 21.

"I don't want to start speculating about whether the samples could have been mixed up, but when they open the samples for the second test we want to be there," he said.

Heras is the first rider to win the Tour of Spain four times after a first win in 2000 before consecutive victories from 2003-2005.

Source: Xinhua