Manchester United and Manchester City will be unable to secure an extension to the Premier League campaign in order to avoid a punishing end-of-season fixture backlog, even if both clubs continue to progress in the FA Cup and European competition, sources have told ESPN FC.
City and United have been forced to postpone this weekend's Manchester derby at the Etihad Stadium due to United's involvement in Sunday's EFL Cup final against Southampton at Wembley and no date has yet been set for the rescheduled game.
City's involvement in the Champions League and United's likely progression to the round-of-16 of the Europa League -- Jose Mourinho's team face St Etienne on Wednesday defending a 3-0 first-leg lead -- could leave both clubs with a fixture headache in the final weeks of the season. Mourinho has already voiced his concern about the number of the games the club have to play between now and the end of the season.
Mourinho's team would be facing six midweek games in the Europa League over the coming weeks should they reach the final in Stockholm on May 24.
And City, who meet AS Monaco in the first-leg of their Champions League round-of-16 encounter on Tuesday, must negotiate a further five midweek ties if they are to reach to the final in Cardiff in June.
United's trip to Southampton on March 12 has also been postponed following the club's progression to the FA Cup quarterfinals, when they travel to Stamford Bridge to face Chelsea.
City, meanwhile, will have to call off their proposed Premier League encounter with Stoke City at the Etihad Stadium on March 11 if they overcome Championship outfit Huddersfield Town in an FA Cup fifth-round replay at the Etihad on Feb. 28.
With fixture problems looming for both clubs, though, sources at the Premier League have told ESPN FC there is no scope for league games to be played beyond the final round of matches on Sunday, May 21.
Barring exceptional circumstances, such as the bomb scare which led to the postponement of United's final day clash against Bournemouth last season, all fixtures must be completed by May 21 to ensure the integrity of the competition.
Pep Guardiola's team had been provisionally due to stage the rearranged Stoke fixture on Feb. 28, but Saturday's 0-0 draw against Huddersfield at the John Smith's Stadium has left City without a date for the visit of Mark Hughes's men in the event of their progression to the quarterfinals.
And despite the availability of a rare free midweek date next week, Southampton and United have chosen not to stage their fixture at St Mary's on March 1.
While there remains space on the calendar for United and City to rearrange their two outstanding fixtures, ongoing progression in the FA Cup and in Europe could pose problems for both and set up the prospect of two midweek games in the final days of the season.
United are due to travel to Burnley on the weekend of the FA Cup semifinals, with City due to face West Bromwich Albion at the Etihad -- dates which would have to be rescheduled if they make it to Wembley.