EXCLUSIVE | By Chelsea correspondent Tony Hughes

Chelsea have made an enquiry about signing Everton defender John Stones, Football Insider sources understand.

The Premier League champions have made a move for the England international as they step up plans to tweak their defence this summer.

But Everton have no intention of selling Stones, who they value at £30million and believe has the potential to develop into the best centre-back in England.

Manager Jose Mourinho has admitted he is in the market for a “defender”, and it is believed Chelsea’s priority at the back is to recruit a centre-half who can be nurtured in a group that includes John Terry, Gary Cahill and Kurt Zouma.

Terry signed a one-year contract extension at the end of last season but turns 35 in December and is in the twilight of his career.

Stones has attracted attention for his polished displays for Everton since joining from Barnsley in a £3m move in January 2013, aged 18.

With four years remaining on his contract, the Merseysiders will resist interest in a player who has developed rapidly during his 15 months in the first team.

He was on stand-by for England’s World Cup squad last summer and is currently one of five full internationals in the England Under-21 squad that takes on Belarus at Oakwell on Thursday evening.