PFA Scotland have accused newco Rangers chief executive Charles Green of "deliberately misrepresenting" the contract situation of departing players in his letter to English clubs.
Green revealed earlier this week he had sent letters to all league clubs in the UK warning them that the players who objected to the transfer of their contracts from Rangers to his Sevco company were in breach of contract.
The union disputes this and insists players are free agents under employment law.
The PFA Scotland's lawyer, Margaret Gribbon of Bridge Litigation, said: "I have been furnished with a copy of Mr Green's letter sent to English clubs in their top two divisions and consider that it amounts to an attempt to deliberately misrepresent my clients' position.
"I have therefore today sent a detailed letter to the clubs setting out the facts and repeating our very firm legal view that Mr Green's allegations of breach of contract are entirely without legal merit.
"My clients are free agents and I am pleased to note that several Premier League clubs have already made applications to the SFA for International Transfer Certificates indicating that they attach little weight to Mr Green's assertion that Sevco is entitled to seek damages for breach of contract."
FIFA will need to settle the disputes with the Scottish Football Association unable to rule on cross-border transfers.
The Ibrox club have announced a deal has been agreed for John Fleck, one of the players who objected, to join League One side Coventry.
But the issue of compensation over transfers involving Steven Whittaker, who has joined Norwich, Steven Naismith, who signed for Everton on Wednesday, and Stoke-bound Jamie Ness will be decided in FIFA's dispute resolution chamber.
However, it is believed the players will be issued with temporary ITCs.
"Furthermore it should also be noted that the SFA has confirmed that in the event of there being a dispute over registration the matter will be referred to FIFA," Gribbon said.
"We can confirm that the SFA has today rejected to issue the registration for one of our clients to an English Premier League club.
"The club in question disputes the rejection and will now write to FIFA seeking their intervention. We expect a speedy response from FIFA." end