The University and College Union faces a legal battle with a group of its own members over its stand on Israel.
Princess Diana's former lawyer has been hired to challenge the legality of a conference motion that his clients regard as a move towards an illegal boycott.
Delegates at the union's annual congress last week voted to ask members to consider the "moral and political implications" of links with the country.
They also agreed to formally investigate Ariel College, a higher education institution in the disputed occupied territories, under the union's blacklisting procedures.
Now Anthony Julius, from Mishcon de Reya solicitors, who acted for the Princess in her divorce and is an advocate for Jewish rights, has written to the union alleging that its actions are anti-Semitic.
He refused to divulge how many members had instructed him, but said: "The only reason the union is saying this is not a boycott motion is because they know that is unlawful. But you can't convert an unlawful act into a lawful one by redescribing it. That's an elementary error not worthy of an academic union."
If the union does not abandon the motion, he said it could face action under the Race Relations Act.
A union spokesman said the motion would be considered by their national executive committee (NEC), which would take advice on its legality.
The union said it had already received advice that discussing the motion at conference was legal.
Tom Hickey, a member of the NEC which proposed the motion, told delegates that teachers and students in Palestine faced "serial humiliation" at the hands of Israel. He rejected claims that criticism of the nation's policies was racist.
"In the face of accusations of anti-Semitism and legal threats, we refuse to be intimidated. We will protect the union from legal threats, but we won't be silenced," he said.
Bill Rammell, the further education minister, also urged members not to boycott Israel, arguing that it would do nothing to further peace in the middle east.
But Sally Hunt, the union's general secretary, said Mr Rammell had misrepresented the motion.