Read Martina Hingis's statement in full

Martina Hingis has quit tennis after admitting she is under investigation for failing a drugs test at Wimbledon this year.

The former world No 1 released a statement this evening in which she said she returned a positive result for cocaine in two urine tests but returned a negative result in a hair test.

Hingis categorically denied taking any banned substance and will not be making further comment for legal reasons.

She said the prospect of a long legal battle to prove her innocence prompted her decision.

Hingis said that she was "shocked and appalled" when she discovered that she had tested positive and said that she had never taken drugs.

She also claimed that her legal team had found inconsistencies in the handling of her test and that Wimbledon officials would not be able to prove that the urine sample taken during the tournament was hers.

She said: "I am frustrated and angry. I believe that I am absolutely, one hundred per cent innocent. The fact is that it is more and more difficult for me, physically, to keep playing at the top of the game.

"And frankly, accusations such as these don’t exactly provide me with motivation to even make another attempt to do so. I attempted a comeback after a three-year break and succeeded in winning three tournaments, bringing my ranking to six in the world. But I’m now 27 years old and realistically too old to play top class tennis.

"So now I’m standing before you, confronting the situation. Today I also informed my sponsors. And so, considering this situation, my age, and the problems I have been having with my hip, I have decided to no longer play tennis on the tour."

Larry Scott, chief executive of the Women's Tennis Association, said the sport's ruling body had not received any official information relating to a positive test from Hingis. He said: “It is important to remember that in the area of anti-doping, all players are presumed innocent until proven otherwise.

"Martina Hingis is a tremendous champion and a fan favourite the world over. In her most recent comeback, she proved again that she can perform at the very highest levels of the game.”

In a glittering career, Hingis won three consecutive Australian Open titles between 1997 and 1999 and was also the champion of Wimbledon and the US Open in 1997. She quit tennis in 2002 after persistent ankle injuries.

Hingis returned to the circuit two years ago and initially showed some of her old form. She then had a relatively disappointing 2007, losing in the third round of Wimbledon and the US Open to Laura Glanville and Victoria Azarenka respectively, both players that she was expected to beat. Before today's announcement, she was No 19 in the world.