Kepa Arrizabalaga was the most overpaid signing anywhere in the world this summer, while Chelsea also lost a potential £20m by letting Thibaut Courtois depart on the cheap, according to a leading data research group.

Chelsea made Kepa the most expensive goalkeeper in history when they signed him for £71.6m last month, hours after confirming the departure of Courtois to Real Madrid.

However, the CIES Football Observatory, which analyses football data, suggest Chelsea wildly overpaid Kepa to the tune of £40.1m - the most over-inflated transfer of the summer.

Chelsea are not the only English club that overpaid in their summer dealings, with Everton's move for Yerry Mina, Danny Ings' switch from Liverpool to Southampton and Bournemouth's deal for Jefferson Lerma among the top-10 over-inflated deals.

The discrepancies are worked out by comparing the actual price paid to the CIES Football Observatory's fair price, which the research group says is the "average between the estimated cost for the most likely buyer before the transfer and the current transfer value".