By Mark Staniforth, PA Sport

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Hurdling great Colin Jackson has slammed drug testing procedures after another positive test heaped more shame on his sport this week.

American sprinter Kelli White - who won the 100m and 200m at the World Championships in Paris this week - has tested positive for the stimulant Modafinil.

Jackson believes the lack of a clear worldwide directive and the grey area over which drugs are legal or not is threatening to make a mockery of athletics.

Jackson told BBC Radio 5 Live's Sportsweek: "What we're seeing now is a whole mess - it is not structured and that is where the problem lies.

"What brings the whole credibility of the sport down is that there are no real answers to anything."

And Jackson singled out the American governing body USTF for particular criticism with White just the latest US athlete to fail a test.

It has emerged that White had previously tested positive for a steroid at a Golden League meeting a year ago - the IAAF did not consider the offence worthy of a ban but the French authorities banned her from competing for six months.

And only four days ago it was revealed that 400m winner Jerome Young tested positive in 1999 but was cleared by USTF and allowed to compete in the 2000 Olympics.

Jackson added: "The problem with USTF is that apparently they don't have the same degree of dope testing as we do in the UK.

"Random dope testing is not taken on board as much as we do and I guess that's why a lot of these things happen."