Chelsea are refusing to relent in the battle to sign Everton’s John Stones and will make an improved bid of around £26m despite Roberto Martínez’s anger over the situation..

The Spaniard has accused José Mourinho of unsettling the 21-year-old England international by going public with an initial £20m offer, which was rejected.

Martínez maintains Chelsea received a letter stating that Everton would not allow Stones to leave and he is unhappy with how the Premier League champions are conducting their business. However, the player is believed to be keen on the move if the two clubs can agree a fee.

Mourinho, it is understood, received the correspondence yet maintains there was no mention of the player not being available at any price. He remains confident of being able to sign Stones, who cost £3m from Barnsley in January 2013. He has four years left on his contract.

Mourinho sees him as the ideal replacement for the 34 year-old John Terry and believes a vast improvement on Stones’s £30,000-a-week contract will help ease a passage to Stamford Bridge for the start of the Premier League season on 8 August.

With Everton sure to demand more than £30m, Stones is set to become the most expensive English defender, eclipsing the £29.1m spent by Manchester United to sign Rio Ferdinand in 2002.