Chelsea reacted with fury last night after being banned from signing any players until January 2011.

The club have been found guilty by Fifa of inducing Gaël Kakuta, an 18-year-old winger, to break his contract with Lens two years ago. They will be unable to buy any players during the next two transfer windows.

In addition, Chelsea must pay Lens £113,000 “training compensation” for Kakuta, who has been suspended for four months. The teenager was also ordered to pay £682,000 compensation, for which Chelsea are “jointly liable”, according to Fifa.

Chelsea have pledged to mount “the strongest appeal possible” and will take their case to the Court of Arbitration for Sport (CAS) in Lausanne if they fail to have the ban overturned by the world governing body.

Their progress will be monitored closely at Old Trafford after Le Havre confirmed yesterday that they are pursuing a similar case against Manchester United. The French club are angry about the way in which United signed Paul Pogba, another French teenager, this summer.

Chelsea will argue that the punishment is disproportionate to the relatively modest fine imposed and the nature of the offence, while senior figures at the club — who have maintained their innocence — believe that they are victims of an anti-English conspiracy at the heart of Fifa and Uefa.

Yesterday’s punishments came two days after Eduardo da Silva, the Arsenal striker, was given a two-match ban for diving by Uefa, leading Arsène Wenger, the manager, to voice similar concerns about Barclays Premier League clubs being victimised.

“Chelsea will mount the strongest appeal possible following the decision of Fifa’s Dispute Resolution Chamber over Gaël Kakuta,” a club statement read.

“The sanctions are without precedent to this level and totally disproportionate to the alleged offence and the financial penalty imposed. We cannot comment further until we receive the full written rationale for this extraordinarily arbitrary decision.”

Lens insisted that Chelsea have got what they deserve, claiming that they effectively stole a contracted player after an illegal approach before his 16th birthday. “The outcome was expected as the kid was under contract and was stolen from us,” Gervais Martel, the Lens president, said.

“Chelsea behaved unacceptably by contacting the player before he even reached his 16th birthday. He’d been training with us since he was 8½. He did have a contract according to French training rules.”

Chelsea claim that Kakuta was playing as an amateur for Lens’ youth academy and, as he was not registered with the French Football Federation, was available to sign for them as a free agent. But at a hearing in Zurich last week, Fifa’s Dispute Resolution Chamber (DRC) ruled against them.

The DRC adjudged that Kakuta had a valid contract with Lens and that he was offered inducements to break it by Chelsea in June 2007.

The details of the DRC’s judgment have not been made public, but when news of Chelsea’s interest in Kakuta first emerged, rumours began circulating in France that he had allegedly been offered a massive signing-on fee and lucrative professional contract.

Chelsea deny this and insist that Kakuta’s first contract was that of a typical first-year scholar, who can expect to earn less than £1,000 a month.

Chelsea were stunned when they received a fax from Fifa yesterday as they had not been expecting a verdict for another three months. The club have been found guilty of making illegal approaches in the recent past and if the DRC’s decision is upheld, questions will be asked about the future of Frank Arnesen, the sporting director, who was in charge of their youth development at the time.

Le Havre are seeking sanctions against United over the signing of Pogba, although the Old Traffrord club claim that they have acted properly. “Of course we are still pursuing our case,” Alain Belsoeur, Le Havre’s managing director, told The Times. “It is a very serious case. This is clearly a message from Fifa. I am aware of the details of the Lens case and ours is similar.”

Both cases would appear to hinge on French law, which prevents young players signing professional contracts before their 16th birthday.

Any attempts by Chelsea to delay the hearing in the hope of having the transfer ban frozen by the CAS, thereby enabling them to buy players in January, are likely to fail as Fifa seems certain to fast-track the appeal.