More frequent international competitions, as well as additional international club games, could lead to major changes in the national leagues, including a reduction in the number of teams competing in them, a shift from weekend to weekday games, and fewer domestic games, the report said.

The report, compiled by consultancies KPMG and FTI's Delta Partners and seen ahead of publication by Reuters, predicted that in addition to cutting club revenue, such a move would hit viewer interest in games.

It would also reduce advertisers' interest in domestic football and put strains on players, leading to poorer play, it said.

Gianni Infantino, president of soccer's world governing body, said last month that FIFA was discussing holding the World Cup every two years instead of the current four-year cycle.

Arsene Wenger, the former Arsenal manager and FIFA's head of global development, has proposed a new calendar which would see a reduction in the number of international breaks for qualifiers, with one or two longer breaks to compress the qualification process.

He suggested holding World Cups in alternate years with continental championships such as the Euros and the Copa America in between. He is also calling for mandatory rest periods to avoid player burn-out.

However, the idea has run into strong opposition from UEFA, which runs the hugely popular and lucrative Champions League, and from national leagues such Italy's Serie A, Spain's La Liga, and England's Premier League, which have an enormous global following.

UEFA president Aleksander Ceferin has argued that a more frequent World Cup would dilute its value and harm other competitions. He has threatened to lead a European boycott of an additional World Cup.

FIFA has not responded to a Reuters request for comment, but it has previously promised its own feasibility study.

The report, which also involves the MLS, Mexico, Japan and South African leagues as well as the European big five, said even the least disruptive options would cost them billions annually.

The biggest hit would be to TV revenues.

It calculated that the 40 biggest domestic leagues around the world and UEFA club competitions like the Champions League stand to lose as much as EUR5 bn ($A7.8 bn) per season in audiovisual deals alone - more than a third of what the leagues make in TV rights today.

The report said commercial losses and match day revenues would drop by about 25%.

It took in various scenarios. But in all of them leagues project that starting from the 2024-25 season, they would be forced to reduce to 18 teams and to finish their seasons in early May to fit FIFA's plans.

Australian Associated Press