A total of five stables - including the yard of top trainer Martin Pipe - have been visited unannounced and blood or urine samples taken from what is believed to be several horses.

The Jockey Club, horse racing's governing body in the UK, will not say why the five stables were picked, but it hopes to have the results of the tests within a week.

Peter Webbon, the Jockey Club's Veterinary Director, said in a statement: "As part of the Jockey Club's policy for testing horses in training, I can confirm that five trainer's yards were visited this morning and that approximately 350 blood samples were collected for testing.

"I fully anticipate that the findings from these tests will back up our belief that drug abuse within British racing is minimal.

"The operation went as smoothly as we could have hoped and I hope that disruption to their training programmes at this busy time was kept to a minimum."

The Jockey Club has carried out regular testing in training since 1998, but Tuesday's operation was the first time that unannounced testing has taken place in Britain.

There is no suggestion that any allegations have been received about Pipe or any of the other four National Hunt stables involved.

However, the Jockey Club's statement says that: "A trainer's yard is selected for testing either at random or on the basis of intelligence received."