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.

The match was initially scheduled for 16:30 on Sunday, 23 October but was moved to the Saturday because of Chelsea's Champions League fixture the following Tuesday.

The Metropolitan Police said that because TV commitments prevented the "high-risk" fixture from starting by 16:45 on the rearranged date - which, for safety purposes, is the agreed cut-off time with London clubs on weekends - an alternative solution had to be reached.

"Our first priority in policing football - and the increasing crime and disorder associated with it - is the safety of fans and those who live and work in the local community," said the Met.

However, a letter to the council sent by MUST's legal representatives highlighted that "there is no history of significant disorder at previous matches" between the two sides, which include matches that have kicked off later than 16:45.

The letter states that if they do not receive a response by 12:00 BST on Friday, 14 October, then MUST "reserve the right to commence judicial review proceedings" against the council "without further notice".

"United fans and football supporters in general have had their fill of being on the receiving end of unfair and irrational decisions by councils and the police, who seem to consider us to be a public order problem rather than ordinary people freely enjoying a day out," said a MUST spokesperson.

"Tickets had been sold for this game and fans are being treated like second-class citizens."

Chelsea Supporters' Trust said it stands with MUST and also criticised the decision.