Barcelona have submitted an official bid for the services of Chelsea defender David Luiz, according to Sky Sports.
Luiz has been targeted by the Spanish champions throughout the transfer window and a formal bid has been made to acquire the Brazil international's services.
The 26-year-old had been given an extended break before returning to Chelsea's pre-season schedule due to international commitments in the Confederations Cup, which he won with Brazil.
Luiz has now returned to the Chelsea set-up but Barcelona have now formalised their reported interest, which will test Mourinho's resolve to keep the player at Stamford Bridge.
Luiz and his Chelsea teammates are still in the United States on the second stage of their tour, and the Brazil international is reported to be happy to be part of the Blues' upcoming campaign under returned manager Mourinho.
Barcelona face further difficulties in trying to attain the player's services as the 26-year-old signed a new five-year deal in September last year.
Mourinho himself has said there is "no chance" the player will be sold, as he has become an integral part of Chelsea's squad since his £21 million move from Benfica in 2011.
Mourinho has spoken highly of the defender recently, saying: "David has come back with us fantastically and we have to use his quality.
"He has big self-confidence and self-esteem, especially when he comes up from the back with the ball.
"Can he improve defensively? Of course. But it is my job to try always to improve players."
However the Spanish champions are eager to add a defender to their ranks - and Luiz, capped 28 times by Brazil is their top target.
If they fail in acquiring his services, newly appointed Gerardo Martino will reportedly look to sign Daniel Agger.
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