Mo Farah has admitted using a drug that is banned by anti-doping authorities and categorised as class-C in the UK, although did so before it was prohibited.

The double-Olympic gold medallist revealed that he had tried Khat whilst visiting Somalia in 2003 during an interview with Piers Morgan.

The stimulant, which is derived from a plant and chewed, became a class-C drug in the UK in 2014 due its increasing use. Possession of the drug can result in up to two years in prison and an unlimited fine. It is not illegal in Somalia.

"I just wanted to test it. It’s a natural plant. It’s different. You chew it and it is like drinking ten cups of coffee," said Farah.

"You think a lot and plan a lot. Nothing happens really and you go home and you cant sleep. It’s crazy, everything is fine when you are doing it but the next day nothing happens.

"It is like wasting time really. Nothing ever gets done."

Farah took the drug at the end of the 2003 season, meaning he was not in 'competition' at the time. Five months later the ingredient in the plant was prohibited by the World Anti-Doping Agency (WADA).

The admission on Piers Morgan's Life Stories, which airs on Friday evening.

The long distance runner, who won double gold at the recent World Athletics Championships in Beijing, came under heavy scrutiny after it was alleged his coach Alberto Salazar and his US team, the Nike Oregon Project, had violated anti-doping rules and doped US record holder Galen Rupp.

Farah himself has not been accused of any wrongdoing.

Asked by Morgan what he thinks of athletes who take illegal drugs.

He said: "It depends what they get done for. If (it's) something that you do, you know what you're doing, then it should be - I think - a longer ban.

"Anywhere I am in the world I get a one-hour slot and testers can come in that hour and if I'm not there it's a missed test - and I believe all countries should be able to do what we're doing. If you're going to compete with me you have to do the same thing."

But asked for his thoughts about the suspicion that successful athletes have taken something illegal, he said: "It does annoy me a lot.

"The whole reason I moved to the US to be coached by Alberto Salazar is to be able to improve one or two per cent.

"I was sick of coming sixth in the world, seventh in the world, and get close to a medal, but not quite there, half-a-second, and that's why I moved to the other side of the world, and my training and a lot of stuff has changed, so people automatically think...sometimes it frustrated me."