Scotland Yard has said it will “enhance” the police presence outside Westminster as fears were raised that serious clashes could take place in the run-up to the key Brexit vote next Tuesday.

Tobias Ellwood, the Defence Minister, denounced the “orchestrated intimidation, violence and abuse” of MPs, which, he said, was being played out under the “very shadow of Parliament, which is such an iconic symbol of democracy”.

The Bournemouth MP, who in March 2017 famously gave first aid to Keith Palmer, the police officer stabbed to death by a terrorist outside the Commons, warned that things were “ratcheting up” and said his concern was that a “minor clash could escalate into something far more serious and that would all [be] played out in front of the world's media".

MPs were issued with fresh security advice, saying: "If a route does not feel safe, do not take it."

Laurence Taylor, the Metropolitan Police’s Deputy Assistant Commissioner, said the force continued to assess if any crimes had been committed in relation to behaviour directed at MPs and noted: “Certainly, leading up to the vote next week, we will be enhancing the policing presence and we will ensure that we have appropriate measures in place to attempt as far as possible to reassure people that they can go about their lives without fear of harassment or alarm."

He said a very clear direction had been given to officers on the ground that if they witnessed criminal behaviour, then there was an “expectation that an arrest will be made”.

The political backlash to the intimidation of MPs came as Theresa May suffered a blow to her no-deal preparations after 20 Tory rebels joined forces with Labour, the SNP and the Liberal Democrats to defeat the Government, by just seven votes, to restrict its tax powers in the event Britain left the EU without an agreement.

To cheers from opposition benches, MPs voted by 303 to 296 to limit the Government's no-deal tax powers. No 10 said such a move would be an "inconvenience rather than anything more significant".

But Jeremy Corbyn, the Labour leader, said: “Theresa May must now rule out no-deal once and for all.”

Earlier, No 10 announced the Prime Minister would make the closing remarks in the Brexit debate next Tuesday in a last-ditch bid to rally Tory colleagues behind her proposed deal ahead of the vote.

The general view at Westminster remains that Mrs May’s Plan looks doomed, a view set to be expressed in today’s first PMQs of 2019.

As tensions mounted ahead of Tuesday’s vote, John Bercow called on the Metropolitan Police to crack down as a matter of urgency on the “toxic attacks” on parliamentarians and journalists outside Westminster.

The Commons Speaker described the attacks, by what he said was a “regular coterie of burly white men,” as a “kind of fascism” given that women and ethnic minority citizens were being targeted.

His intervention came after there was an outcry at the intimidation suffered by Anna Soubry, the Conservative backbencher and leading Remainer, who was barracked by a group of men as she walked from a TV interview on the green outside Parliament to the House of Commons.

In a letter to Cressida Dick, the Met Commissioner, Mr Bercow said it was “intolerable” that MPs and journalists had to run the gauntlet of “being ritually insulted, abused, intimidated, threatened and harassed”.

Referring to the attack on Ms Soubry, the Speaker noted how schoolchildren on a visit to Westminster were at risk of witnessing “these toxic attacks”.

Earlier, the Tory MP, who represents Broxtowe in Nottinghamshire, said that while she accepted a level of criticism and abuse as a parliamentarian, she, nonetheless, expected the authorities to act when it "crossed the line".

"It crossed the line in December…[when] journalists were being attacked. Faisal Islam[from Sky News], who is male, was racially abused by these people; it's the same group, all on video, and the policy of the Metropolitan Police is to ignore it," she declared.

In a separate development, No 10 slapped down Stephen Barclay, the Brexit Secretary, who sparked outrage from Tory MPs after suggesting a People’s Vote should be avoided if they wanted to end such abuse from protesters, which had shown “how divisive this process has been”.

Mrs May’s spokesman made clear that calls for a second poll and intimidation from pro-Brexit campaigners were "separate issues".

Labour’s Chuka Ummuna, a leading People’s Vote campaigner, denounced Mr Barclay’s remarks as “disgraceful,” tweeting: “The notion we should be intimidated into NOT holding democratic votes is deplorable.”