As well as writing to its members — a group that the A.F.A. said accounts for collective annual revenues of 500 million pounds (more than $615 million) — the group also has written to FIFA to warn it that the agents would begin legal action within seven days if the proposals were not abandoned.

“We cannot accept any regulations that provide for capping of our fees or restrict our freedom to act for any party in a transaction,” the A.F.A. wrote to FIFA. It described the regulations as “unlawful and anti-competitive.”

According to data released by FIFA, agents earned more than $2 billion in the five years from 2014 through 2018, a figure that dwarfed the amount paid to teams as part of a separate so-called solidarity mechanism designed to reward youth development programs for their roles in producing players.

Stein, the A.F.A. head, accused FIFA of “breathtaking arrogance” for failing to engage with his group before making a final decision on its new rules, and suggested the excesses of soccer leaders were worse than any found in his industry.

“Have you been to their offices?” he said of FIFA. “All that marble and gold, that’s taking money out of the game.”

Controlling agents’ growing influence in the global marketplace has been a struggle for FIFA since the value of the transfer market — and the prices for top players — began to grow exponentially amid a television revenue boom that began in the 1990s. As part of the proposals FIFA will consider next month, it will take responsibility for licensing agents, a role it gave up about a decade ago. At the time, it acknowledged that the task, on a global scale, was beyond its capabilities.

Peter Kenyon, a former chief executive of the Premier League giants Manchester United and Chelsea, suggested actors in the soccer industry would be able to find ways of getting around the new rules, as they have done when it comes to other forms of regulations. He noted that it was not agents who determined the fees paid for players, but clubs.