Manchester City have suspended Kolo Toure following Football Association confirmation that the defender has failed a drugs test.
An 'A-sample' from the Ivory Coast international tested positive for a 'specified substance' and the case is now the subject of a legal process.
Sky Sports News sources understand that Toure tested positive after the Manchester derby on 12th February for a substance contained in a dietary supplement.
The 29-year-old is therefore set to miss at least Saturday evening's Premier League match against Wigan at Eastlands.
Roberto Mancini's City then face next week's Europa League first leg with Dynamo Kiev and the FA Cup quarter-final versus Reading.
A statement on City's official website read: "Manchester City confirm that the FA has informed Kolo Toure that an 'A-sample' provided by him has tested positive for a specified substance.
"As result of this, he has been suspended from participating in all first-team and non-first-team matches pending the outcome of the legal process.
"There will be no further comment from the football club at this stage."
The World Anti-Doping Agency defines a specified substance as one that is "more susceptible to a credible, non-doping explanation".
The WADA punishment for such a positive test ranges from a warning to a two-year ban.
Official FA doping control regulations state: "The definition of Specified Substances now allows for the exercising of greater discretion in disciplinary cases.
"This affords a Regulatory Commission the opportunity to impose a shorter suspension where an offence relates to a Specified Substance."
The regulations also state: "If the substance is a Specified Substance the decision as to whether to impose a provisional suspension will be at the discretion of The Football Association."
Hamilton midfielder Simon Mensing recently served a four-week ban after he tested positive for a 'specified substance'.
The 28-year-old tested positive for the stimulant methylhexaneamine at the end of January and subsequently missed five matches after a period of ineligibility was imposed on him.
Mensing maintained he was unaware he had ingested the substance while using a dietary supplement - an explanation which was accepted by UK Anti-Doping.
Toure joined City for £14million in 2009 to mark the end of a seven-year association with Arsenal.
He had been signed by Arsenal manager Arsene Wenger from Ivorian side Mimosas and he went on to captain the Gunners on a number of occasions.
Toure also served as City skipper under Mark Hughes until he was succeeded by Carlos Tevez for this season.