Police raided 10 homes and and found banned doping substances during the raids with 13 people being detained.

Quick Step spokesman Alessandro Tegner said the assistant was questioned but was later released by police.

"To make it clear, this is not a Quick-Step case, but a Belgian doping case dealing with amateur cyclists," Tegner told Belgian television.

A police spokesman said: "A large quantity of doping products has been found. I cannot confirm any names."

One of those questioned will appear before a judge on Friday charged with possessing banned substances, two were allowed to go home while a decision has yet to be taken on the remaining 10.

The investigation began in response to information received from a Belgian member of parliament who had claimed that he had evidence of doping in the country.

Quick-Step also denied that their former world champion Tom Boonen had been affected by the raids.

"I have just spoken to Tom Boonen a few moments ago and, as far as he was concerned, there have been no raids on his house," said Tegner.

"I have also spoken to some other riders and they have been training and unaware of anything going on. We will have to wait and see what the prosecutors say."

Earlier this year Quick-Step were thrown into controversy when former team rider Johan Museeuw was charged with doping offences.