The Muslim Council of Britain is urging UK Prime Minister Theresa May to treat an inquiry into MP Boris Johnson remarks on Muslim women's dress seriously, and not to "whitewash" the investigation.
Former Foreign Minister Boris Johnson sparked outrage earlier this month after he described Muslim women wearing burqas as looking like "letterboxes" and "bank robbers".
He is currently being investigated for alleged Islamophobia, though the UK's largest Muslim body fears that he may get away with his offensive remarks.
"We are hopeful that the party will not allow any whitewashing of this specific inquiry currently in process. No one should be allowed to victimise minorities with impunity," the letter said.
The former top diplomat, who resigned in July over the UK's Brexit plans, has been branded a "pound shop Donald Trump" over his comments against the religious dress.
The remarks were made in his weekly column for the Daily Telegraph, where he said he opposed a ban on the niqab, but made a series of disparaging comments about the face covering, worn by some Muslim women.
"If you tell me that the burka is oppressive, then I am with you," he wrote.
"If you say that it is weird and bullying to expect women to cover their faces, then I totally agree - and I would add that I can find no scriptural authority for the practice in the Koran."
He then claimed: "It is absolutely ridiculous that people should choose to go around looking like letter boxes."
Any female student who turned up to school "looking like a bank robber" should be asked to remove their face covering, he then added.
But he stopped short of calling for a ban on face coverings in public, as introduced in some European countries.
Boris Johnson has been linked to far-right American activist Steve Bannon, which some believe could be behind the former foreign minister's remarks on the niqab.