England's refereeing body are set to investigate how a rule was apparently breached during Carlisle United's epic penalty shoot-out defeat at Derby.

Professional Game Match Officials Limited (PGMOL) are looking into the handling of Tuesday's EFL Cup marathon, which Derby eventually won 14-13.

The focus is on why ref Darren England did not instruct a Carlisle player to withdraw from the shoot-out after Derby midfielder Bradley Johnson was deemed unable to take part.

The EFL are satisfied Derby themselves did not break any rule by advising the fourth official midway through the shoot-out that Johnson - who ended the game with an ankle injury - could not take a penalty.

At that point Carlisle had to nominate a player to withdraw from the shoot-out to even up the numbers.

But although their nominated player, keeper Mark Gillespie, did not take a penalty for the Blues, he was allowed to remain in goal to face all Derby's 16 spot-kicks.

That amounted to a rule breach - and it is this the decision by ref England and his colleagues that will come under scrutiny.

There is no suggestion that United sought to gain any illegal advantage or will face any sanction, while there is no sign that either club is pursuing the matter with the authorities.

PGMOL's probe, though, follows Keith Curle's post-match call for "clarity", with the shoot-out having been briefly delayed whilst the matter was being debated by the officials.

An EFL spokesman last night told the News & Star : "Under the laws of the game, any player that is on the field at the final whistle, or receiving treatment, is eligible to take a kick.

"In this particular case, Derby were trying to get him [Johnson] ready, and they kept the fourth official informed.

"But at 8-8 [in the shoot-out], it became clear that he wouldn't be able to participate, and they informed the officials - and that is within the laws of the game.

"Carlisle nominated the goalkeeper, which means he had to leave the field of play - but he didn't.

"Technically there has been an infringement.

"The PGMOL are dealing with the issue and will discuss it in the next 24-48 hours."

Law 10 in the FA's rules and governance section states: "If, at the end of the match and before or during the kicks, one team has a greater number of players than its opponents, it must reduce its numbers to the same number as its opponents and the referee must be informed of the name and number of each player excluded.

"Any excluded player is not eligible to take part in the kicks."

Derby manager Pearson said he understood Carlisle's concerns at the time, but stressed Johnson was injured and the Rams had done nothing wrong.

"He [Johnson] was unable to take part in the penalty shoot out which caused a little bit of unhappiness in the technical area, but I'm not paid to manage the game," said Pearson.

"Carlisle weren't over-pleased with that but do you expect a player who is injured to take a penalty?

"It's one of those situations where I understand their perspective and it's up to the officials to deal with that and, eventually, they did."

Curle had said he was "disappointed" at the timing of Johnson's withdrawal.

After the game the United boss said: "I would have thought, once you go through extra-time, you then nominate your penalty takers but also nominate the people who can’t take penalties.

"I could have said to my players - 'I don't want you to take a penalty, be injured'.

"That’s what the confusion was. I think there should be some clarity on that."