Tell Mama, which records hate crimes, said there was a “direct link” between the former foreign minister’s comments and an uptick in incidents targeting women who wear the niqab – which covers the face and hair apart from the eyes.
At least four women have been called “letterboxes” in public since Mr Johnson ignited a row over Islamophobia with an article published in the Daily Telegraph on 5 August.
In the previous week, no incidents targeting women who wear face veils were reported to Tell Mama – although there were several involving those wearing headscarves.
But women wearing the niqab were targeted on 8 August in London, followed by another three incidents in London and Luton the next day, and a fourth in the capital on Friday.
Rabina Khan, a councillor from Tower Hamlets, said one of her friends was called a “letterbox” while boarding a train.
“One of the women was wearing a niqab and she was the last one to get on the train. The man laughed and said: ‘Hold on, you forgot the letterbox.’
“Freedom of speech should not extend to being highly offensive to someone who has done nothing more than step onto a train for an enjoyable day out.”
Fiyaz Mughal OBE, the founder of Tell Mama, said that women who wear the veil are known to “rarely report any anti-Muslim hate since they become desensitised to it through being targeted many times”.
“There is a direct link with Mr Johnson’s comments and an impact on visibly Muslim women as a whole,” he told The Independent.
“Many of these women are from black and minority ethnic communities and we know from the work of Tell Mama that in many cases, racial and anti-Muslim hate intersect.
“Mr Johnson thinks his flippant comments were funny, and while his comments were about the burqa, perpetrators see any visibly identifiable woman and off they go with their bigotry and prejudice.”
Mr Mughal said the Tory MP’s comments had not only “dehumanised” Muslim women, who he also compared to bank robbers, but “emboldened mainly male perpetrators to have a go at visible Muslim women as a whole”.
Research shows that reported Islamophobic attacks mainly target women who wear clothing signalling their faith, and Tell Mama warned last month that incidents were being fuelled by “political discourse” that emboldens perpetrators.
Police found that Mr Johnson’s comments did not meet the threshold for a hate crime investigation but the Conservative Party has launched an investigation over possible breaches of its code of conduct.
A ComRes survey found 53 per cent of British voters were opposed to punishment for the former foreign secretary, against 40 per cent who said he deserved to be disciplined.
Mr Johnson has returned to the UK from a holiday in Italy amid an escalating Tory civil war over the incident, with claims four cabinet ministers have privately expressed dismay at the way it has been handled.
Arch Brexiteer Jacob Rees-Mogg denounced the investigation as a “show trial” motivated by the prime minister’s personal rivalry with a man many see as her likely successor, while fellow Conservative MP Andrew Bridgen warned of “open warfare” if he was barred from a future leadership contest.
But Lord Cooper, a Conservative peer and former director of strategy for 10 Downing Street, said: “The rottenness of Boris Johnson goes deeper even than his causal 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.”
Donald Trump’s former chief strategist Steve Bannon, who was the chairman of alt-right website Breitbart News, was in contact with Mr Johnson during a recent UK visitwhere he worked to set up a new Europe-wide populist alliance called The Movement.
Mr Bannon has urged him not to “bow at the altar of political correctness” by apologising, while the US president undermined Theresa May by saying Mr Johnson would make a “great prime minister” last month.
Damian Green, who was Ms May’s de facto deputy, said he feared Mr Johnson was “being turned into a martyr by the alt-right” in what would be “a disaster for him and the Conservative Party”.
Far-right groups including the pan-European Generation Identity, Anne Marie Waters’ For Britain party and Tommy Robinson’s aides have been vocally supporting Mr Johnson’s comments in recent days.
Tell Mama recorded a 30 per cent rise in Islamophobic street incidents and a 16 per cent rise in 2017, taking the number of verified reports to a record of 1,201.
Its annual report warned of a “marked shift” towards more serious offline incidents like physical attacks, vandalism and abuse, as hatred continues to spread on social media.
Researchers said: “Events which stimulate public discourse on immigration and Islam can correspond with a demonstrable ‘spike’ in anti-Muslim hate crimes and incidents.
“It is vital to note that these events are not the underlying cause of anti-Muslim incidents, but rather act as triggers, where people with latent racial prejudices feel emboldened to act on their views, violently or otherwise.”
Victims were predominantly women of Asian ethnicity, while 72 per cent of perpetrators where their identity was known were white men and boys.
Last month a report by HM Inspectorate of Constabulary found that despite “concerted efforts” some police forces were still responding poorly to hate crime by failing to properly identify incidents, or inform and support victims.
A spokesperson for Mr Johnson declined to comment.