The UK's largest Muslim organisation is calling on Theresa May to ensure the inquiry into Boris Johnson's burka comments is not a "whitewash".
According to the Guardian, the Muslim Council of Britain will tell the prime minister "no-one should be allowed to victimise minorities with impunity".
Mr Johnson is facing an investigation after saying women in burkas looked like "letter boxes" or "bank robbers".
He did not respond to the row in his latest newspaper column.
The Muslim Council of Britain's letter, seen by the Guardian, said it was "hopeful" that the party "will not allow any whitewashing of this specific inquiry currently in process".
The council previously said the support shown to Mr Johnson from Tory MPs had "shone a light on the underbelly of Islamophobia" within the party.
The organisation added that it had received Islamophobic hate mail referring to former foreign secretary Mr Johnson's comments.
Meanwhile, the Tell Mama project, which monitors anti-Muslim violence, reported an increase in incidents of abuse aimed at women wearing the niqab or hijab over the past week.
'Nothing to say'
Mr Johnson - who returned home from his holiday in Italy over the weekend - has not yet responded to the row.
Earlier, he refused to answer the questions of journalists waiting outside his Oxfordshire home, saying he had "nothing to say about this matter" except to offer out cups of tea.
The Conservative Party launched its disciplinary investigation after receiving dozens of complaints about Mr Johnson's remarks.
Mrs May and Conservative chairman Brandon Lewis have called on him to apologise.
But others - including Brexiteer backbencher Jacob Rees-Mogg and comedian Rowan Atkinson - have expressed support.
According to a ComRes poll for the Sunday Express, 53% of people who responded believe Mr Johnson should not be disciplined for his comments.
The complaints against him are being looked at by an investigating officer, who can dismiss them if they are found to be obviously trivial, lacking in merit or unable to be fairly investigated.
Communities and Local Government Secretary James Brokenshire told BBC Breakfast: "There is an ongoing investigation in relation to complaints that have been made over Boris's comments last week. I think that's the right approach."
He added "I would certainly have not chosen the words that Boris used," but would not comment on it further during the investigation.
Should they be upheld, the complaints will then be looked at by an independent panel which could refer Mr Johnson to the party's board, which has the power to expel him.
On Friday, in a separate investigation into a complaint, the UK's equalities watchdog said Mr Johnson's remarks were "inflammatory and divisive" and his comments risked "vilifying Muslim women".