Chelsea are facing a third Fifa probe in eight years over the signing of players under the age of 18.
Fifa have confirmed they are looking into the transfer activity of the Premier League champions in the way they bring promising youngsters to their academy.
The world governing body have refused to reveal the exact nature of any alleged wrongdoing, but they are looking closely into the work Chelsea have been doing.
A statement said: “We can confirm that an investigation is ongoing.
“We cannot provide any details concerning the matters under investigation.”
FIFA rules prohibit the international transfer of players under the age of 18 unless their parents have emigrated for reasons not connected to football or both the player and club are based within 50 kilometres of a national border.
The only other exception is for transfers within either the European Union or European Economic Area (EEA), where the minimum age is 16.
In those cases, clubs must still ensure that the player continues his education and studies for an alternative career, as well as making sure he has a high living standard.
Chelsea have one of the most successful youth programmes in Europe and have won the Uefa Youth League – the under-19 competition including that year’s senior Champions League clubs – twice.
The Blues have also won the last FOUR editions of the FA Youth Cup, clinching the title six times in the last eight years in a stunning dominance of youth football in England.
But some have questioned the transfer fees and signing-on fees paid by the West London club – although they are not the only club to do so.
And despite having one of the best academies in the world at Cobham, Chelsea have a questionable record of blooding first-team players.
But it is the third time the club have been investigated by Fifa, having got found themselves in trouble following the wrangle over French youngster Gael Kakuta.
The club were banned from signing players for two transfer windows initially, before Chelsea managed to overturn the ruling on appeal.
Fifa also looked into the transfer of Bertrand Traore to Chelsea last year, amid suggestions he had been pictured playing for the club before his 16th birthday.
Chelsea have insisted they comply with all Fifa statutes and regulations in response to the investigation.
Atletico Madrid are currently in the middle of a transfer ban until January over the signing of young players.
They even appealed Fifa’s decision to the Court of Arbitration for Sport, but it was upheld by the court.
Real Madrid have also been punished in the recent past and had their ban cut from two transfer windows to one.
Liverpool and Manchester City have both been rapped by the Premier League over the tapping up of youth players.
A Blues spokesman said: “Chelsea FC complies with all Fifa statutes and regulations when recruiting players.”
The West London club could play Welsh youngster Ethan Ampadu in this week’s Carabao Cup clash against Nottingham Forest, but they have reportedly not yet agreed a compensation fee with Exeter for the 17-year-old.