Manchester United Supporters' Trust say they will take legal action unless more away fans are allowed when the team plays at Chelsea later this month.
They want Hammersmith and Fulham Council to issue a new safety certificate to allow the full allocation of 2,994 travelling supporters.
United's allocation has been reduced to 2,370 because of policing concerns.
MUST said "fans are being treated like second-class citizens".
A spokesperson added: "For a Supporters Trust to engage lawyers is always a last resort, but enough is enough."
The Premier League confirmed last week that the match will kick off at 17:30 BST on Saturday, 22 October.