Everton defender John Stones has handed in a transfer request in an attempt to force through a move to Chelsea, sources have told ESPN FC.
The request, submitted on Tuesday, is set to further test Everton's resolve to keep a player they have repeatedly insisted is not for sale.
Everton manager Roberto Martinez said on Friday that the club would not sell Stones even if he asked for a transfer because it was too late in the window to lose such a key player.
The club have moved over the last few days to secure another centre-back, with Argentina international Ramiro Funes Miro expected to complete a £6 million move from River Plate if his work permit application is successful.
Martinez, though, has indicated that he has not moved for Mori in anticipation of losing Stones, who has a contract at Goodison Park until 2019.
Asked on Friday if a transfer request from the England international would make Everton more willing to sell, Martinez said: "No. When you've got a long-term contract, those things doesn't change at all.
"I think the situation has gone too far for any action to change it, because it is not something that is brand new.
"If someone comes in from nowhere and puts in a transfer request, that is a different situation. In a saga of I don't know how many weeks now, we are very clear."
Chelsea have had bids for Stones of £20m, £26m and £30m rejected, but reports suggest they are set to make a fourth offer, while Manchester United have also been credited with an interest.
The 21-year-old, who has four England caps, joined Everton from Barnsley for £3m in January 2013.