In an emotional televised interview, Stevens said: "I was tested for a prohibited substance. It is not a performance-enhancing substance.
"It is a very serious substance and something over a period of time I have realised I have an issue with, something I think has basically ruined my life up until this point.
"It is pretty distressing talking about this. When you think how much time and effort so many people have put in to my career - I have thrown it away with irresponsible behaviour."
He added: "I want to say I truly am very sorry. I want to change my life and hopefully one day get the faith back that people had in me."
Under World Anti-Doping Agency regulations, Stevens could be facing a suspension of up to two years.
But for a first offence, if a player can fully establish the use of such a specified substance was not intended to enhance sport performance, the sanction could range between a warning and a one-year ban.
He has been caught which in the long run will be to his benefit
Former Bath and England centre Jeremy Guscott
In December, Manchester prop Marshall Gadd was suspended for two years after testing positive for a banned substance.
And in 2006, Australia winger Wendell Sailor was banned for two years after testing positive for a recreational drug.
Stevens can request for his B-sample to be tested to confirm the A-sample result, ask for a judicial committee to rule whether there has been an anti-doping violation or accept an anti-doping violation has occurred and go before an independent judicial hearing.
"He is going to need the greatest support from his family and friends," former Bath and England centre Jeremy Guscott told BBC Radio 5 Live.
"He has been caught which in the long run will be to his benefit. He has come out and admitted his problems and is seeking help and is getting counselling.
"It's not brilliant PR for rugby, but the message that has to go out is that he is seeking help."
Professional Rugby Players' Association chief executive Damian Hopley said he was shocked by the news.
"This is very out of character for Matt," he said.
"He had built a reputation as a fine ambassador for the game, who does a tremendous amount of voluntary work for a number of charities and good causes.
"We will do everything we can to provide Matt with the support he needs at this very difficult time."
Stevens, who has 32 England caps, joined Bath in September 2002 after representing South Africa at Under-18 and Under-19 levels.
He made his England debut in the non-capped game against the New Zealand Barbarians in December 2003 and claimed his first two caps on the 2004 summer tour against the All Blacks.
He played in all of England's games in the 2007 Rugby World Cup in France.
Away from rugby, he is best known as the runner-up in a celebrity version of 'The X Factor' in 2006, raising £125,000 for the Nelson Mandela Children's Fund in the process.
The England squad are set to fly out to a training camp in Portugal on Sunday.
Johnson could call veteran Leicester prop Julian White into the squad, while Worcester's Matt Mullan, Gloucester's Nick Wood and Newcastle's David Wilson are also in the Saxons squad.
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