President Aleksander Ceferin, 49, told German broadcasters ARD that swift action was needed to slow down the growing imbalance between rich and poorer clubs - although he didn't believe it could be fully stopped.

Ceferin spoke of "a deciding moment" after Brazil's Neymar went to Paris Saint-Germain for a world record €222 million (A$332 million) and Kylian Mbappe potentially joining the French side next year on a full transfer for around €180 million (A$268 million).

"Financial Fair Play was started to raise stability in football. And it was successful. But the times are changing," Ceferin said.

"We have to adapt it, modernise it, and we must do something for the balance of the sporting competition."

Financial fair play was set up by UEFA to force clubs not to spend significantly more money than they generated.

UEFA is investigating PSG amid growing criticism of huge transfer fees which are also paid in England thanks to both a new major television deal and new investors pouring large amounts of money into clubs.

Ceferin told ARD that UEFA was not afraid to sanction big clubs if necessary.

He said UEFA could limit the number of registered players at a club, cut the number of loan deals and let the transfer window end in late July ahead of the start of the season, instead of late August.

"Football is a fantastic product," he said. "The high transfer sums don't come out of nowhere.

"But we must preserve competitiveness. We don't want to have the gap become so big that only a few clubs can play serious football in the end."