By Chelsea correspondent Tony Hughes

Chelsea will make a fresh bid of around £25million for John Stones within days, Football Insider sources understand.

The Premier League champions’ opening offer of £20million for the England defender has been rejected by Everton.

But Jose Mourinho has not been put off and has sanctioned an improved bid for the player, who has emerged as Chelsea’s prime remaining target in the summer window.

Despite Everton Roberto Martinez insisting that Stones is not for sale, Mourinho, who is at Chelsea’s pre-season training camp in Montreal, said: “At the moment they say: ‘We don’t accept this bid’. They don’t say: ‘We don’t accept bids; any bids’. While they say, ‘We don’t accept this bid’ until Sept 1, we can improve the bid: £1 more, £10 more, we can improve the bid.

“If one day Roberto Martinez or the chairman says: ‘He’s not for sale; we don’t accept any bid’, game over. So during this process, it’s clear, we don’t deny we made a bid for Stones. It means we are trying to buy.

“We made a bid, we can make it — it’s an open market until Sept 1. We didn’t make an offer on Sept 2 or in March or two days before we play against Everton. We are making the bid in the period we have to make it, which is the open market.”

Martinez has criticised Chelsea’s public pursuit of Stones, which has echoes of Mourinho’s attempts to prise Wayne Rooney from Manchester United two summers ago.

Everton’s insist that Stones is not for sale but Chelsea remain determined to make Stones the most expensive English defender in history. The current record is the £30 million that Manchester United paid Leeds United for Rio Ferdinand in 2002.

Mourinho regards Stones as the ideal long-term candidate to replace John Terry as the team’s defensive leader.

Football Insider revealed in June that Chelsea had made an enquiry for the centre-back, who is widely regarded as a future cornerstone of the national team.

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 do not want to lose 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 year and Ferdinand Tweeted on Saturday that he regards Stones as the best young centre-back in the country ahead of Michael Keane and Eric Dier.