Criminal abuse and harassment of MPs are running at unprecedented levels, reflecting “polarised opinions” in the aftermath of the Brexit vote, Britain’s most powerful police officer told a parliamentary committee on Wednesday.

The warning from Cressida Dick, the Metropolitan police chief, was accompanied by official statistics showing that the number of crimes committed against MPs had more than doubled to 342 in 2018 from 151 the year before.

Giving evidence to parliament’s human rights committee, Dick said: “We do believe that the current context is unprecedented”. There had been “a very considerable rise” in the number of threats MPs faced in the course of their work, she added.

“What we see is that polarised opinions are having a big impact on the scale and nature of protest activity…and other behaviours as well,” Dick said in evidence to the joint committee of MPs and peers.

The Met chief was speaking a day after it emerged that police are investigating comments by Ukip candidate Carl Benjamin, speculating about whether he would rape the Labour MP Jess Phillips.

Hours afterwards Phillips had said a man “ran down the street” alongside her as she left Westminster at around 5pm on Tuesday, asking why somebody “shouldn’t be able to joke about my rape” online.

Although Dick was not asked directly about the case, committee chair Harriet Harman did press the police chief about the harassment of MP Anna Soubry outside parliament in December and January by aggressive pro-Brexit protesters. Change UK MP Soubry has been repeatedly targeted and threatened in the past year.

Harman complained that video footage of the Soubry incident showed “your police officers standing by and not intervening”. She asked Dick: “What did you think when you saw that footage? ‘Oh, that’s good. He’s getting his chance to protest?’”

Dick said she could not comment on the details of the case, but admitted she believed that policing outside parliament at that time “was overall too passive”. She said she had since asked 60 officers to take responsibility for safety outside Westminster with the power “to intervene if there is crime apparently committed”.

The committee chair said she believed that “something had gone wrong with our law and order” if MPs like Soubry could no longer be interviewed outside parliament. “If MPs can’t be interviewed on College Green, that’s bad for our democracy,” Harman added.

Harman said that MPs also faced increasing hostility that wasn’t always criminal. She told the committee that Joanna Cherry, an SNP MP, had told her that “she couldn’t walk across Westminster bridge without people blocking her way after a vote” and that she was advised “you should take a taxi at public expense”.

Neil Basu, an assistant commissioner at the Met, added that crimes against MPs had continued to rise in the first four months of 2019, with 152 crimes reported, meaning that “somewhere in the region of 450 crimes” were expected this year.

A small number of MPs were disproportionately targeted, Basu added. He said that 10 MPs accounted for 29% of the crime reports, with Dick adding that “people from minority communities and women” were the most affected.

Max Hill, the director of public prosecutions, highlighted to the same committee examples of successful prosecutions of people who had threatened MPs, even where the abusers were anonymous. In April a 51-year-old from Luton, Jarod Kirkman, was sentenced to 42 weeks in jail after sending anonymous threatening emails to seven MPs, including Nicky Morgan, Yvette Cooper and Heidi Allen. Morgan said that the malicious messages were death threats relating to Brexit.

Separately, the head of the top civil servants’ union will on Thursday accuse politicians – including the Brexiter Mark Francois and the remain supporter Lord Adonis – of deliberately undermining trust in public servants to advance their political causes.

Dave Penman, head of the FDA union, will tell the union’s conference on Thursday that the UK is witnessing the systematic undermining of the principles of a professional and impartial civil service in the fight over Brexit.

He will tell delegates that attacking named mandarins is “the weapon of choice for many in the European Research Group (ERG), Leave Means Leave and assorted fellow travellers”.

“We even witnessed cries of ‘shoot him’ from the audience at the mention of the name Olly Robbins – the prime minister’s Europe adviser – during a speech by the ERG’s Mark Francois.

“Shame on Francois, shame on his enablers in the ERG and shame on all those politicians who have indulged in ‘traitor’ tropes to progress their narrow political agenda.

“On the left we have seen the likes of Lord Adonis accuse the civil service of being the enablers of Brexit and establishment lackeys who’ll frustrate a future Labour government.

“Left and right are coalescing around a narrative which suits their ideological objectives, but undermines the foundations of an impartial civil service.”