Employees at the web offset printer were told yesterday that a wage cut of between 10-20% would be needed across the company in order for it to continue as a self-supporting entity.
A secret ballot has been arranged to decide whether employees are willing to accept the pay cut.
However, Unite national officer Steve Sibbald told PrintWeek that the company had no right to legally cut wages and the union was writing to members advising them not to accept the change.
He said: "We understand they are setting up an unlawful adhoc collective arrangement, but that will have no legal bearing at all on changing terms and conditions.
"The only way they can do it is through a collective agreement with a recognised union. Even if one person decides not to accept it in a secret ballot, legally they have the right to refuse to take the cut."
A legal expert, who did not wish to be named, said that, negotiating without a union was a "grey area", depending on what route BGP takes after the ballot has taken place. He added: "I am sure the company will have taken legal advice before making this decision."
Sibbald added that he expected many employees not to accept the cut, particularly those nearing retirement.
He said: "If you are 63 or 64 you aren't going to take a pay cut, you are going to say 'no thanks, I'll take redundancy'. Those people aren't going to agree to this.
"The problem in the sector isn't wages, it is over-capacity. Everybody at BGP, Polestar and Walstead could take a 50% pay cut, but that isn't going to create more work. At the moment the web-offset sector is just subsidising publishers at the cost of their staff."