Chelsea are under investigation by Fifa for a third time in eight years over a potential breach of the regulations that relate to the signing of foreign players under the age of 18.

The Premier League champions have said they comply with “all Fifa statutes and regulations when recruiting players” but the world governing body has been alerted to alleged impropriety and says it is looking into it.

“We can confirm that an investigation is ongoing,” Fifa said in a statement. “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 50km of a national border.

The only other exception is for transfers within either the European Union or European Economic Area, 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.

Atlético Madrid are under a transfer ban for breaching Fifa rules over the signing of minors. They appealed against the decision but it was upheld by the court of arbitration for sport. Real Madrid were also punished last year but they had their ban reduced from two transfer windows to one.

Chelsea have been investigated twice previously over their signing of minors. They were banned from signing players for two transfers windows in 2009, over their conduct in the deal that took the attacking midfielder Gaël Kakuta from Lens to Stamford Bridge in 2007, although the penalty was overturned on appeal.

They were also investigated over the transfer of Bertrand Traoré, the Burkina Faso forward, after pictures emerged of him playing for the club when he was 16 before he had international clearance. Traoré signed for Chelsea after his 18th birthday.

Manchester City and Liverpool have been punished by the Premier League this year for breaches of the rules regarding the signing of domestic academy players. Both of them were found to have tapped up young players.

City were banned from signing academy players for two years and fined £300,000. Part of the ban – from June 2018 – will come into effect only if the club reoffend in the next three years. Liverpool were banned from signing academy players for two years – the second year suspended for a three-year period. They were fined £100,000.