The proposed format change is the latest innovation to the world's oldest cup competition after the FA announced that from this season onwards, all quarter-final ties would be finished on the day of the original tie instead of going to a replay.

And it follows in the footsteps of the Copa America, who also trialled the extra substitution in the final of this summer's tournament - though neither Chile nor Argentina took advantage.

FA chief executive Martin Glenn said on www.thefa.com: "With the cup now adopting a straight knock-out format from the quarter-finals onwards, the introduction of a fourth substitute in extra time will bring extra intrigue and interest.

"The Emirates FA Cup is renowned for its great history and tradition but, as the game's governing body, it is important the FA continues to look at how it can further add to the drama and spectacle of a competition loved by millions around the world.

"Also, from a technical point of view, it will be interesting to see how managers use the chance to make an additional substitution in such high-profile games and the impact it has on the final result.

"Player welfare and being mindful of the number of games people play at the elite level has also been a consideration."

Manchester United are current holders of the competition after their extra-time 2-1 win over Crystal Palace in May.

They had defender Chris Smalling sent off in the additional period, having made all three of their substitutions in normal time.

This year's tournament begins in earnest next week with extra preliminary rounds.