A Tory peer and former aide to David Cameron accused Boris Johnson of “moral emptiness”, casual racism and “courting fascism” as division over the former foreign secretary’s comments about Muslim women threatened to develop into a full-blown crisis for Theresa May and her party.

The comments by Lord Cooper, a former pollster who worked for Cameron at No 10, came after Jacob Rees-Mogg, leader of the party’s anti-EU right wing, criticised the prime minister for backing an investigation into Johnson’s remarks, in which he compared women wearing burqas to “letterboxes” and “bank robbers”. Rees-Mogg said May had countenanced a “show trial” of Johnson because of her “personal rivalry” with him.

As different wings of the party split into rival camps, amid rumours that Johnson is preparing a leadership bid in the autumn, new evidence emerged suggesting that a series of hate crime attacks on Muslim women had been perpetrated as a direct result of Johnson’s remarks in his Daily Telegraph column last Monday.

An abnormal spike in anti-Muslim abuse aimed at women wearing the hijab and niqab has been recorded by the government-backed hate crime monitoring group Tell Mama.

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In a tweet, Cooper offloaded the frustrations of the party’s anti-Johnson wing, which mainly consists of people who opposed the UK leaving in the EU in the June 2016 referendum. After Johnson, who returned from holiday abroad on Saturday, refused to apologise, Cooper said: “The rottenness of Boris Johnson goes deeper even than his casual racism and his equally casual courting of fascism. He will advocate literally anything to play to the crowd of the moment. His career is a saga of moral emptiness and lies; pathetic, weak and needy; the opposite of strong.”

A close friend of Johnson told the Observer that he was “as likely to ride naked down Blackfriars cycle lane waving an EU flag” as he was to apologise. His father, Stanley, writing in the Sunday Telegraph today, said that his son was “spot on” in his comments on the burqa, but should have gone further and called for a ban in certain circumstances.

While Johnson was deluged with support in the letters pages of the Daily Telegraph, further criticism of him came on Saturday from the Muslim Council of Britain (MCB), which said support for him from Tory MPs and others “shone a light on the underbelly of Islamophobia” in the party.

In a statement, Harun Rashid Khan, the MCB’s secretary general, said: “The impact of Boris Johnson’s comments is real and worrying, and indicates the importance of a full, transparent and independent investigation into his conduct, in particular given the lack of action in previous cases of Islamophobia in the party.”

'He is as likely to ride naked down Blackfriars cycle lane waving an EU flag [as apologise]' Close friend of Johnson

Johnson is, according to senior Tories, unlikely to lose the whip or face official censure as a result of the internal investigation – though he may be asked to take diversity training, an offer he is unlikely to accept without a fight.

No 10 is concerned that the row could do lasting damage to the Tory party’s relations with the Muslim community, as well as enthuse Johnson’s support base for any potential leadership contest. The majority of niqab-wearing victims who have called Tell Mama’s helpline since the article appeared said the perpetrator either used phrases such as “letterbox” or referred to Johnson. One victim said she was considering leaving the UK after detecting increased hostility post Johnson’s intervention in the debate on face veils in public.

In the week before Johnson’s comments, Tell Mama reported no incidents against women wearing a niqab. Yet, following his comments, a London woman wearing a niqab reported being abused on Wednesday; with another three incidents reported in the capital and Luton the following day, and another taking place in London on Friday. Anti-Muslim abuse directed at women wearing the hijab also rose significantly with five incidents reported in Guildford, Leicester and London on the day that Johnson’s column was published, equalling the total number of incidents reported during the entire week before Johnson aired his views on face-covering veils in public places. Another two incidents involving hijabis were reported on Wednesday, followed by annother on Thursday and another on Friday.

One of the victims, who lives in the West Midlands, said: “Johnson’s recent comments have made me feel much more on edge and it has definitely made me fearful of even going to work or to walk in town without my husband or someone who can stand up for me. “Myself and my husband are considering moving to an Arab country where we would feel more comfortable and do not have to put up with abuse.’

Iman Atta, director of Tell Mama, said it was no surprise that Johnson’s comments had motivated individuals to commit crimes against Muslim women.

Atta said: “Comments and political statements have impacts. Boris Johnson is a privileged white male who has had a route to power and influence that many from black and ethnic minority groups could not even dream of. In this privileged position he calls Muslim women were wear the burqa, ‘letterboxes’ and ‘bank robbers’. “These are some of the most marginalised women, who cannot find employment and in some instances, have few choices.