Fifa’s decision to move its legal department to Miami will cause “major disruption” with almost two-thirds of staff not relocating to the US, employees at world football’s governing body have told the Guardian.

But Fifa has dismissed concerns that cases, including several involving high-profile sexual abuse allegations, will be delayed as a result of the high turnover of staff and denied there was a backlog.

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The Guardian understands almost 80 members of the 120-strong legal department based at Fifa’s headquarters in Zurich will have left their posts by the time of the move to new offices in mid-August. Among the departments moving to Coral Gables are the ethics committee, which investigates serious allegations of misconduct, and the audit, compliance and risk management teams.

It is understood that most senior figures, including the chief legal and compliance officer, Emilio García Silvero, have committed to the move but the vast majority of employees opted to leave after being given a month to decide whether they wanted to go to the US, which will co-host the expanded 48-team 2026 World Cup. That has left Fifa scrambling to find replacements, with about 50 positions left to be filled in the legal department. Many of the roles, including a head of corporate legal, were advertised on the job networking site LinkedIn last week.

“There are several people with a lot of experience who are leaving,” said one source who is departing after 10 years at Fifa. “The system is already struggling to cope with so many cases and this is going to cause major disruption.”

An Inside World Football report last month quoted a Fifa insider who described the situation as “a sinking ship” and said the overpromotion of lawyers to replace those departing had caused “problems and chaos”. Fifa has insisted the move will not have an impact on operations but it is understood there is an acceptance the office in Miami will have considerably fewer staff when it opens.

Sources have told the Guardian they have concerns there are not enough people to investigate allegations of sexual abuse, with some cases such as the charges brought against three youth coaches in Gabon in 2021 still awaiting a verdict.

Fifa’s leadership is understood to be adamant it has enough resources to manage the vast number of cases presented to the ethics committee every year, even though this year’s total is believed to have already surpassed last year’s. It points to the introduction of Fifa’s first independent “integrity experts” last year – about 20 in total – who are helping to support investigations and there will be more focus on bringing cases to a conclusion once the switch to Miami is completed.

A Fifa spokesperson said: “The move of the legal and compliance team to Miami, which was announced last September, is ongoing and will be effective this summer. There are no plans to downsize this Fifa team as part of this move, and it does not impact operations. As with any restructuring of this kind in a global company, the process involves the departure of some staff and the hiring of new staff in the new location.

“The whole process has also been monitored by the independent Fifa governance, audit and compliance committee, which has been periodically informed about the different steps of this process. Fifa strongly rejects any claim that this move would impact judicial investigations handled by the Fifa disciplinary or ethics committee. There is currently no ‘backlog’ of judicial or dispute procedures, and each case is handled in an appropriate timeline based on its specific circumstances.”