Three of Maria Sharapova's major sponsors have distanced themselves from the tennis superstar following her admission of a failed drug test.

Nike acted swiftly, suspending its contract with the five-time grand slam champion, with watchmaker Tag Heuer and luxury car brand Porsche following suit.

Tag Heuer has severed ties altogether, with a statement reading: "Maria Sharapova was under contract with TAG Heuer until December 31th, 2015. We had been in talks to extend our collaboration.

"In view of the current situation, the Swiss watch brand has suspended negotiations, and has decided not to renew the contract with Ms Sharapova."

Porsche signed Sharapova as its first global endorser in 2013, and said in a statement: "We are saddened by the recent news announced by Maria Sharapova.

"Until further details are released and we can analyse the situation, we have chosen to postpone planned activities."

The Russian has been the highest-earning female athlete in the world for the last 11 years, with her off-court endorsements dwarfing her income from prize money. According to Forbes, the 28-year-old earned US dollars 21million in 2015.

Sharapova, 28, revealed on Monday that she had tested positive for meldonium following her quarter-final defeat against Serena Williams at the Australian Open in January.

Meldonium, which Sharapova said she had legally taken throughout her career, was placed on the banned list by the World Anti-Doping Agency (WADA) at the beginning of the year following ''evidence of its use by athletes with the intention of enhancing performance''.

Sharapova claimed she did not realise the substance was illegal, but took ''full responsibility'' for her actions.

The International Tennis Federation (ITF) said Sharapova had been informed of the positive test on March 2 and she will be provisionally suspended from March 12.

A statement from sportswear giant Nike read: "We are saddened and surprised by the news about Maria Sharapova.

"We have decided to suspend our relationship with Maria while the investigation continues.

"We will continue to monitor the situation."

Former British number one Annabel Croft believes if Sharapova is given a two-year ban or more for failing a drug test it could spell the end of her career.

Croft told BBC Radio Five Live: "The game moves on constantly and it's very, very difficult to keep up at the best of times.

"It's not to say never as she's one of the best competitors out there. She's utterly ruthless as a competitor and she's got a strong will and a strong mind.

"It's not to say she couldn't come back at some level but I think it would be very difficult if it was a two-year ban or more than that to get back to the heights she's at."

Sharapova, though, remains hopeful she can return to the sport after serving her punishment.

The ex-world number one said: "I let my fans down, I let the sport down that I have been playing since the age of four and I love so deeply.

''I know with this I face consequences. I don't want to end my career this way and I really hope I will be given another chance to play this game.''

Sharapova left her home in Russia aged seven to train at the Bollettieri Academy in Florida. Coach Nick Bollettieri was shocked by Monday's revelation and is convinced it was an innocent mistake.

He told BBC Radio 4: "I'll tell you why I'm surprised because, throughout her entire career, she's always been above board in everything.

"I don't think Maria Sharapova would continue doing something, especially being in the limelight, if there was something that she knew about. I would say that this is a mistake, but she accepts the consequences.

"I hope and pray that they will look back at the record she's established and hopefully they will give her the sanction but allow her to play again. Certainly she does not want to go out of tennis this way."