The City 3rd choice keeper was loaned to Luton earlier in the season and has played just once, the FAW Premier Cup tie at Welshpool, for Cardiff City. Because Dundee Utd would be his third club the move was blocked.
The switch was vetoed by the Scottish Football Association but furious United boss Craig Levein said: "I've taken it to Fifa to give an answer on it.
"I am really annoyed about this.
"David went on an emergency loan from Cardiff to Luton but because they are in the same league structure English clubs have an agreement that it doesn't count as a transfer.
"Because he had already played for Cardiff in a cup tie it has been blocked and our friends at the SFA have refused to recognise the English agreement.
"It's a crazy rule and I don't for the life of me understand it.
"The paperwork has been done before the window closed so if we get the go-ahead it can still go through."
Should he go Forde will join Matt Green, Steve MacLean, Jason Byrne, Kevin Cooper, Chris Gunter and Kasper Schmeichel who have left in January (Schmeichel of course ended his loan move and Cooper's contract was terminated).
Cardiff's only incoming player was loan keeper Peter Enckelman.