United have revealed they will sell tickets to Gers supporters independently for their fifth-round showdown at Tannadice on February 2.
The Light Blues had refused their ticket allocation when they were drawn to play their former SPL rivals due to resentment between the clubs.
Rangers chief executive Charles Green believes the shock move by United could jeopardise the safety of fans who do decide to attend the cup-tie.
And John Macmillan (right), the general secretary of the Rangers Supporters' Association, today appealed to his fellow-fans not to break the boycott.
Macmillan has also called on the SFA to step in and prevent United from going down a route which many fear could spark a riot on Tayside.
He said: "I am in total agreement with Charles Green and the club board in not taking tickets, and I am also very supportive of his words at the weekend about this.
"I would urge Rangers fans not to go to Dundee. I have even heard suggestions that Rangers fans will go up just to stand about. I would urge them not to even consider that either.
"This is a potentially explosive fixture and I think Dundee United are being a bit irresponsible here with their actions. I think the SFA should ask Dundee United to seriously reconsider selling tickets to Rangers fans themselves for this game."
Macmillan fears it will be impossible for United to determine if people who buy tickets for the Rangers section are genuine supporters or potential troublemakers.
He added: "The bottom line is that if someone is standing in an area designated for Rangers supporters then they are going to be considered one. But who can prove they are or they aren't?
"I would hope Rangers fans would go along with what the board and the football management team want and don't go to the Dundee United game."
Macmillan has also defended the controversial decision by Rangers fans to boycott the fixture – and accused their Dundee United counterparts of "double standards".
He said: "This club was very vocal in its condemnation of Rangers in the summer and in their assertion that they didn't need us in the SPL. Of course, they are perfectly entitled to that view, but now they are holding out their hand for money.
"Furthermore, it wasn't that long ago that the Arab Trust and Federation of United Supporters' Clubs condemned Rangers fans for bringing football into disrepute because we didn't take tickets.
"But these are the fans who threatened that they wouldn't buy season tickets if Rangers were allowed back into the SPL. For me, that is double standards."
Macmillan added: "Our stand has been not to accept tickets for this game from day one. I'm glad to say that at long last we have got someone who is listening to us.
"Charles Green did listen to the fans, I can assure you of that. Word got to him through Ally McCoist that the fans weren't wanting to take tickets.
"I was of the view that although we wanted to not take tickets if Rangers had taken an allocation as a club, then fans would have got those."