According to reports, the Blues are also reluctant to allow fellow defender Cesar Azpilicueta to complete his move to the Nou Camp in what is becoming an increasingly fractious and complicated set of negotiations.
It comes after Barcelona swooped for £55million Sevilla defender Kounde even after Chelsea had seen their offer accepted.
Chelsea have blocked Marcos Alonso’s transfer to Barcelona after the Spanish club snatched defender Jules Kounde from under their noses after Sevilla had accepted a £55m offer
Chelsea are likely to block Barcelona’s bid for Cesar Azpilicueta if Kounde joins the Catalans
Barcelona are increasingly likely to snatch Jules Kounde away from Chelsea with a late swoop
Andreas Christensen has already made the move from Chelsea to Barcelona this summer, but there is now doubt over whether fellow defenders Alonso and Azpilicueta will follow.
Spanish newspaper Diario Sport report that tensions between the two clubs have increased following the Kounde deal.
Chelsea believed they had secured the France international after Sevilla agreed to their £55m offer but Barcelona made their move to gazump them at the eleventh hour.
Barca had pulled off a similar move to hijack Chelsea’s move to sign Raphinha from Leeds United earlier in the summer.
Chelsea came close to signing Kounde, 23, last summer and earlier in this window there was a strong belief in Spain that a move to Stamford Bridge was inevitable once the Todd Boehly takeover was completed.
Alonso has featured on Chelsea’s pre-season tour to the US despite Barcelona’s interest
Chelsea manager Thomas Tuchel made clear his irritation at Barcelona’s pursuit of Cesar Azpilicueta over the weekend, hinting he may block the transfer
However, the player’s preference is to join Barcelona and the club even briefly had his shirt name available for purchase on their online store. It was quickly deleted.
But Chelsea have reacted with anger and are reportedly willing to block the exits of Alonso and Azpilicueta to the Nou Camp.
They have managed to sign Napoli’s Kalidou Koulibaly this summer to reinforce their defence, which has also seen Antonio Rudiger depart to Real Madrid, but will either have to chase other targets beyond Kounde or keep existing players.
Chelsea manager Thomas Tuchel had already strongly suggested that Barcelona’s move for Azpilicueta may be blocked.
Asked at a press conference in Orlando ahead of their 4-0 defeat by Arsenal whether he was annoyed by Barcelona’s approach, Tuchel replied: ‘Maybe a little bit.’
Chelsea have reinforced their defence with the signing of Kalidou Koulibaly from Napoli
He added: ‘It is a tough question because I am not sure if I want to give Azpi what he wants.
‘At some point, it is about what we want. I just made the comparison about how much we fought for Koulibaly, who is a national team player and roughly the same age.
‘He is a hugely important player for Napoli, but we have a Spanish international who is the captain at Chelsea. I see him maybe at the same level, but Barcelona don’t see him on that level.
‘So I am not sure if I want to give him what he wants as he is a huge player.
Raphinha is now at Barcelona after they beat Chelsea to the punch to sign him from Leeds
‘We don’t think so much about other clubs. The focus is on us and what we need. I said that to him and I say it to you that I understand on a personal and a career level.
‘But I am not only in this role to give him what he wants. I am a manager for Chelsea who wants to do what’s best for Chelsea.
‘He doesn’t like it, but he understands. It is tough for him because the other club is permanently on him.’
It all adds to a sense of frustration at Stamford Bridge, with Tuchel ripping into his players’ lack of commitment as they were thrashed 4-0 by their London rivals in their final tour match.