Teams competing in the latter stages of this season's FA Cup will be able to use a fourth substitute in extra-time, the Football Association announced on Friday.
The English FA last year agreed to play a lead role in testing the idea, which was proposed by football's law-making-body the International Football Association Board (IFAB).
As part of a two year trial, the scheme will now be used for first time in the quarter-finals, semi-finals and final of this season's FA Cup.
"With the Cup now adopting a straight knockout format from the quarter finals onwards, the introduction of a fourth substitute in extra time will bring extra intrigue and interest," FA chief executive Martin Glenn said in a statement.
"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."
Chelsea host Manchester United, Arsenal face Lincoln, Tottenham take on Millwall and Manchester City meet Middlesbrough in the quarter-finals next weekend.