Manchester United, Arsenal, Southampton and Atletico Madrid watched Celtic forward Moussa Dembele during Wednesday's Champions League clash with Barcelona, sources close to the Scottish club have told ESPN FC.
Dembele has been a revelation for Celtic this season, scoring 16 goals in 26 games in all competitions, including a hat trick in the 5-1 win over Rangers in September.
The 20-year-old has also netted an impressive three goals in five Champions League games, while on the international scene he has broken into the France under-21 side, scoring four goals in four games, including a double against England U21s earlier this month.
Celtic beat off interest from a number of Premier League clubs to sign Dembele from Fulham in the summer, but the player's suitors have continued to keep tabs on his progress in Scotland.
Sources have told ESPN FC that United and Arsenal have been long-term admirers of Dembele and have watched him on several occasions this season, while Southampton have now joined the list of clubs -- who signed Victor Wanyama and Virgil van Dijk away from Celtic in recent seasons -- are also tracking him.
Atletico sent scouts to watch Dembele in Celtic's 2-0 home loss to Barca this week after being alerted to his impressive form.
Celtic, though, are desperate to keep hold of him and are unlikely to consider a sale in the January transfer window.