• Club can expect transfer ban if guilty of breaking regulations • Chelsea say they complied with rules and most were trialists

Chelsea are set to hear from Fifa whether they will be banned from the transfer market after being investigated over potential rule breaches with regard to more than 100 foreign players under the age of 18.

The governing body has overseen a long-running case against the club, which was started by the compliance unit of its transfer matching system (TMS). It flagged 25 player cases at the beginning of last year which it thought warranted further scrutiny.

Fifa’s disciplinary committee has since conducted phase two of the investigation – a deep dive into how Chelsea have registered players at their academy over a number of years. The club responded to a request for information by opening their books to the committee which, in turn, highlighted a high number of cases to examine.

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The committee is now considering its final decision, meaning Chelsea should know soon whether they will face any sanctions. If found guilty, a transfer ban would be expected. The club would be able to appeal to Fifa and then to the court of arbitration for sport (Cas).

The basic rule is that clubs cannot bring in players under 18 from other countries, 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. Under Fifa’s article 19, the only other exception is for transfers within either the European Union or European Economic Area where the player is aged between 16 and 18.

Chelsea contend that the majority of the 100-plus player cases involved short-term triallists at their academy, who did not go on to sign for them. They have argued there is no law against trialling under‑18 international players; that it is common practice across Europe and, as such, they believe these cases ought not to be included in the final judgment. In others that led to them making signings, the club maintain they fully complied with Fifa’s rules.

The investigation – the first of its kind into a Premier League club – has been complex, with a high degree of communication between Chelsea, Fifa and the Football Association. One of the key questions has been where the line is drawn between a player being on trial or spending time as a resident at the club academy.

Chelsea did not want to comment but they referred to a statement from last November in which they said they had “fully cooperated with Fifa and provided comprehensive evidence demonstrating its compliance with the applicable Fifa regulations”.

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The club have signed Christian Pulisic this month from Borussia Dortmund for £58m, although he will not join them until the summer, and they have taken the striker Gonzalo Higuaín on loan from Juventus until the end of the season, with options to extend the loan or sign him permanently. Chelsea deny that the moves are in reaction to the threat of a transfer ban.

Fifa, which says it cannot comment on ongoing investigations, stepped up its attempts to regulate transfers by introducing TMS in 2010 and its pursuit of these cases springs from a determination to help protect children from potential exploitation and trafficking.

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Fifa has investigated Barcelona, Real Madrid and Atlético Madrid for infringements related to the signing of minors and each of the Spanish clubs were given a two-window transfer ban, although Real served only one after a successful appeal to Cas. Chelsea are among five Premier League clubs currently under investigation and there are more worldwide.

The French investigative website Mediapart reported last November as part of the Football Leaks expose that Fifa would seek to impose a four-window ban on Chelsea, together with a fine of 500,000 Swiss francs (£383,000). It also said that the club’s alleged offences related to the recruitment of 19 minors, including 14 who were illegally transferred. It was after this that Chelsea made their November statement. It is possible that Mediapart’s player numbers were obtained at an early stage in the investigation, most likely at TMS. Alternatively, they could relate to Chelsea’s most serious alleged transgressions. The Guardian understands that TMS pushed for Atlético to receive a four-window ban, but the disciplinary committee has the final say on sanctions.

Fifa has established a principle which says a systemic breach of the rules should entail a transfer embargo, irrespective of the number of minors involved. The amount of cases does affect the severity of the sanction. It is also understood that Fifa worries that a one-window ban does not create harmful consequences, and can be worked around with sufficient planning.

Facebook Twitter Pinterest Chelsea have denied breaking rules over the signing of Bertrand Traoré, pictured here in 2016. Traoré now plays for Lyon. Photograph: Glyn Kirk/AFP/Getty Images

Fifa considered 183 player cases in its final judgment against Atlético; 153 would be confirmed. Atlético’s numbers were inflated, in part, by their partnership with Dalian Wanda Group, a Chinese property company, that led to many Chinese players coming to train at their academy. Barcelona were found guilty of violating the rules on the registration of 31 minors; in Real’s case it was 39 of 70 investigated. Cas upheld 37 of the guilty verdicts.

The severity of the infringements is key and Fifa deems there to be different levels, with the most serious being a breach of article 19. It is unclear how many of the alleged Chelsea cases would fit into the higher end of the scale, although it is believed to be a significant number.

Chelsea would reject that, but one that would appear to do so is that involving Bertrand Traoré, a forward now at Lyon, who was signed to professional terms by Chelsea on 1 January 2014, the first available day after his 18th birthday. Pictures emerged of him playing for the club against Arsenal in a “non-competitive” game on 23 October 2011, when he was 16. The pictures came to Fifa’s attention in January 2016 and it is believed that they were the trigger for the investigation.

The club have argued that they had an option agreement, registered with and approved by the FA and Premier League, to secure Traoré’s registration after he turned 18 and which allowed them to play him in what were effectively trial games.

Mediapart has claimed that Chelsea paid £154,000 to Traoré’s mother and £13,000 to his former club AJEB to take him from Burkina Faso in April 2011. It has reported that Fifa decided Chelsea had “paid for the player to go to an English [private] school while he was underage, and incited him to be transferred to England and to be paid for it”. Again, Chelsea said they were cooperating with Fifa and had complied with the rules.