Players will be able to represent two different clubs in the same European cup competition from next season after governing body Uefa changed its rules.
As it stands, January signings cannot play for a second club in the Champions League or Europa League if they have already featured in that competition.
A fourth substitute will be allowed in extra time of knockout ties, while 12 substitutes can be named in the finals.
Some Champions League group games will kick off earlier - at 17:55 BST/GMT
Registering new players:
A club may register three new eligible players without any restrictions after the group stage.
"This is in line with the existing regulatory situation in the different domestic leagues, which does not impose restrictions on the eligibility for competitions of players registered for a new club during the winter transfer window," said Uefa.
This season, Barcelona are not able to select Philippe Coutinho for their Champions League knockout ties because he featured earlier in the competition for Liverpool.
Likewise, striker Pierre-Emerick Aubameyang cannot play for Arsenal in their Europa League campaign because he played in the competition for Borussia Dortmund.
However, the Gunners are able to pick Henrikh Mkhitaryan because he played in the Champions League, not the Europa League, for Manchester United. The same applies to Alexis Sanchez, who moved to United in a swap deal with Mkhitaryan.
As it stands, all Champions League games kick off at 19:45 BST/GMT, although Uefa can make exceptions to this rule.
Staggered kick-off times will be introduced as standard next season.
In the group stage, there will be two matches played on Tuesday, and two on Wednesday, kicking off at 17:55 BST/GMT.
All fixtures on the last matchday will be played simultaneously.
The rest of the matches, including all the knockout ties, will kick off at 20:00 BST/GMT.
Europa League matches from the group stage until the last 16 will kick off at 17:55 and 20:00. The quarter-finals, semi-finals and final will all kick off at 20:00.
A fourth substitute may be introduced in knockout matches - but only during extra time. This will not affect the other three substitutions.
Teams contesting the Champions League and Europa League finals, plus the Uefa Super Cup, can name a matchday squad of 23 players instead of the 18 allowed in the rest of the competition.
"This gives clubs and, in particular, coaches more flexibility regarding substitutes, and facilitate their squad management for the most important fixture of the season," Uefa said.
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