Its call comes after the move to cast the 47-year-old actor in the role of Jinnah, the Muslim who is revered by Pakistanis as the founder of their nation, caused consternation among Asian and Afro-Caribbean actors, who disagreed with the idea of him "blacking up" for the role.
If Irons were to accept the part, it would revive the debate over whether it is politically permissible for white actors to black up, while black actors find it desperately hard to get work.
While Laurence Olivier put on coatings of black make-up to play Othello at the National Theatre, and the Black and White Minstrel Show appeared weekly on television in the early Sixties, sensitivities are now far more acute.
Sir Alec Guinness suffered critical derision when he blacked up for David Lean's film of A Passage To India, and American Equity tried, albeit unsuccessfully, to ban the British actor Jonathan Pryce from playing a Vietnamese spiv in the Broadway production of Miss Saigon, when there were Asian actors available to play the part.
The prevailing climate against blacking up has caused its own problems.
Shakespeare's Othello is rarely performed at the National Theatre or Royal Shakespeare Company, and white actors are unable to perform one of the greatest tragic roles in the English language.
But the heads of those two companies believe that this as a small sacrifice compared with the insult to black actors and black audiences of seeing a white man put on black make-up.
The Jinnah project has been undertaken by Professor Akbar Ahmed, a fellow of Selwyn College, Cambridge who sees Irons as ideal for the part.
Yesterday, Ian McGarry, Equity's general secretary, said that the union disagreed that such a high-profile Asian role should go to a white actor.
"Equity's policy is advisory on the entertainment industry and means that we would expect the producer of this film to have searched as widely as possible for an Asian performer to play the role of Mohammed Ali Jinnah," said Mr McGarry.
"We do not know whether Jeremy Irons has accepted this role, but in any event, we do not believe that this proposed casting could be considered to be made under 'limited and exceptional circumstances' ".
Professor Ahmed, who is co-producing and co-scripting the film, defended his position yesterday, saying that the casting of Irons in the role was intended to help the non-Pakistani community to "understand what a great man Jinnah was" and to boost the self-image of the Pakistani community in Britain.
He said: "Jinnah gives me, as a Pakistani, a sense of identity, territory and, above all, a sense of destiny because he created a Muslim nation which does not exist in history. Jeremy Irons is perfect for the role and if he takes it, it will do a great deal of good for the self-image of the Pakistani community in Britain.
"By seeing a British actor in the role, it will help the non-Pakistani community to see where we came from and what we're all about."
The script of Jinnah has been sent to Irons, but his agents, Hutton management, were unsure if the actor had had time to read it.
"We know he has got the script," said a spokeswoman yesterday. "But as far as we know he hasn't had a chance to read it because he has only just got back from holiday."
The film, due for release next year, to coincide with the 50th anniversary of Indian and Pakistani independence, is due to be shot in Karachi, Lahore, Delhi and Bombay. It is understood that a number of British, American, Pakistani and Arab investors have shown interest in putting up the pounds 2m needed for the making of the film, but only if a well-known international star takes the lead role.Reuse content