Mason Mount and Fikayo Tomori will not be allowed to play for Derby County against Chelsea when the clubs meet in the Carabao Cup fourth round later this month, sources have told ESPN FC.

The pair, on loan at Pride Park from Chelsea for the season, have impressed under the tutelage of Blues legend Frank Lampard and shone as Derby eliminated Manchester United on penalties in the third round at Old Trafford.

Article 6.3 of the EFL rules governing the Carabao Cup allows for loan players to be registered if the parent club provides "written consent," but Chelsea see no reason to make an exception to what is considered standard practice on loan agreements between English clubs.

All Chelsea's domestic loanees have it written in their contracts that they cannot play against the Blues, and last season Tomori sat out, along with Ola Aina and Michael Hector, as Antonio Conte's team beat Hull City 4-0 in the FA Cup fifth round at Stamford Bridge.

The only recent exception to this Chelsea policy is Thibaut Courtois, who helped Atletico Madrid eliminate the Blues from the Champions League semifinals in April 2014.

At the prompting of Jose Mourinho, the club sought to have him declared ineligible, but their appeal was dismissed by UEFA.

Mount has been one of the stars of the Championship so far -- registering three goals and an assist from midfield in 10 appearances to help the club to ninth in the table -- and some have called for him to be included in Gareth Southgate's next England squad.

"Mason Mount can do anything he wants, he is that good," Lampard said in September. "That's not to stay he will go in and be an England starter in their next game, as it's Gareth Southgate's choice.

"But whatever level you put him in, because of his quality, work ethic and mentality -- he wants to get better and better -- he will do anything he wants to.

"It's my decision to play him every week, and it will be Gareth's decision, if and when, to pick him for the England team."