Metropolitan Police chief Sir Ian Blair has ordered an inquiry after a Muslim constable was excused from guarding the Israeli embassy in London.
Sir Ian Blair ordered the review after reports in the Sun newspaper
Sir Ian says he wants an "urgent review of the situation and a full report".
The Sun newspaper said the officer was reassigned on "moral grounds" as he objected to Israeli actions in Lebanon.
But the Association of Muslim Police Officers said it was a "welfare issue" - the officer had Middle Eastern relatives and felt unsafe on that duty.
The officer, who has been named as Pc Alexander Omar Basha, is attached to the Scotland Yard's Diplomatic Protection Group.
He has a Syrian father and a Lebanese wife.
During the summer, when Israel was involved in a month-long conflict with Lebanese militants, the Association of Muslim Police Officers said Pc Basha had asked to be excused from his duties because he felt "uncomfortable and unsafe".
Superintendent Dal Babu, from the association, told BBC News the officer's reassignment had nothing to do with politics but was an "issue around the welfare of a particular officer".
Supt Babu said Pc Basha was now back on diplomatic protection group duties and that "if an incident happens at the Israeli embassy he will deal with it".
Supt Babu accepted that excusing officers from assignments because of moral beliefs would be unacceptable.
"I think that we're going down a very, very slippery slope if we then start having postings based on individual officers' conscience," he said.
"We can't pick and choose."
Lord Mackenzie, former president of the Superintendents' Association of England and Wales, said the move sounded like "a step too far".
"What we don't want is a situation where one particular section of the community is given special reasons for not performing duties because that will simply alienate the rest."
A Scotland Yard spokesman said it would sometimes consider a special request to be moved on moral grounds - but added it reserved the right to post an officer anywhere.
The Metropolitan Police Authority, which oversees the force's work, pointed out that police officers took an oath of allegiance.
The authority, which has also asked for a report, said officers often had to undertake duties where the subject conflicted with their personal beliefs.
But MPA member Peter Herbert said the row was a "ridiculous fuss about nothing" and attacked Sir Ian over an "unwise judgement" on opting so quickly for a review.
"From a security point of view, the Met would be seriously criticised if this guy has relatives in Lebanon and his picture was used around the world to demonstrate the irony about having a Muslim defending the Israeli embassy in the UK."
Glen Smyth, chairman of the Metropolitan Police Federation, which represents rank-and-file officers, said just one two-hour slot outside the embassy had been affected.
The officer had not refused to do duties and had made a simple request which it was "fairly sensible" to grant, Mr Smyth said.
Lord Janner, former president of the Board of Deputies of British Jews, said it was a "grave error" to allow a police officer to avoid his duty.
The Israeli embassy in London, meanwhile, said it was confident the Met Police would find "a satisfactory solution to this particular problem".