They followed it up by recruiting another of the world's most highly sought-after players, Kylian Mbappe, on a season-long loan deal from French champions Monaco on Thursday.

That deal comes with what was described as an option - but is thought to be an obligation - to then purchase the player next summer for a fee of around €180m, which would currently be the second-highest fee ever paid for a player.

UEFA's Financial Fair Play (FFP) regulations were introduced by its former president Michel Platini in a bid to ensure clubs lived within their means and only spent in proportion to their revenue.

European football's governing body released a statement on Friday which read: "The investigatory chamber of the UEFA Club Financial Control Body has opened a formal investigation into Paris St Germain as part of its ongoing monitoring of clubs under Financial Fair Play (FFP) regulations.

"The investigation will focus on the compliance of the club with the break-even requirement, particularly in light of its recent transfer activity.

"In the coming months, the investigatory chamber of the UEFA Club Financial Control Body will regularly meet in order to carefully evaluate all documentation pertaining to this case.

"UEFA considers Financial Fair Play to be a crucial governance mechanism which aims to ensure the financial sustainability of European club football.

"UEFA will make no further comments on this matter while the investigation is ongoing."