Monaco vice-president Vadim Vasilyev has questioned Kylain Mbappe's loan to Paris Saint-Germain in a move he calls "the most successful in world football".

The 18-year-old striker moved to PSG last Thursday in a season-long loan deal with a 180 million euro buy-out clause attached, making him the second most expensive player of all time behind new teammate Neymar.in a season-long loan deal with a 180 million euro buy-out clause attached, making him the second most expensive player of all time behind new teammate Neymar.

"We (Monaco) could not understand to the very end whether this deal was really possible as UEFA had questions to PSG about their compliance with financial fair-play," Vasilyev told Russian news agency TASS.

"It was a very tough deal," he added. "Initially, we did not want the footballer to strengthen our opponents.

"But I think this deal appears to be the most successful one in the world of football."

PSG secured Mbappe less than a month after signing Brazilian superstar Neymar from Barcelona in a world record 222 million euro ($264 million) deal.

Last week Europe's football ruling body UEFA opened a "formal investigation" into PSG's dealings over the transfer window.

UEFA said it would look at whether PSG had violated the financial fair play regulations "in light of its recent transfer activity".

UEFA president Alexsander Ceferin insisted on Monday that "Nobody is above the law".

The Slovenian, speaking to the European Clubs Association in Geneva, added: "UEFA is at your side to ensure that the participants in our competitions are all treated equally.

"Financial fair play is a tremendous success because it has enabled clubs to reduce their losses drastically and adopt an economic model that is viable for the long term.

"Nevertheless, UEFA's credibility and that of football rests on us ensuring that the rules that are in force are respected."rests on us ensuring that the rules that are in force are respected."