Pro-Brexit Tories should risk a long delay to leaving the EU rather than bow to pressure to back Theresa May’s “poisonous” deal, says the QC advising them.

Conservative MPs who sank the agreement in January have been told to stand firm in next week’s repeat “meaningful vote” – even if the price is an extension of the Article 50 exit notice until the end of 2020.

Significantly, the advice comes from Martin Howe, a barrister specialising in EU law appointed by anti-EU Tories to analyse the legal implications of any changes to the Irish backstop secured by the prime minister.

If the advice is followed, it would ensure a second defeat for the deal, probably next Tuesday – and pave the way for MPs to demand an Article 50 extension later next week.

Until now, the suspicion has been that Brexiteer Tories will be cowed into backing a deal they detest for fear of a long delay to departure that would put the entire project at risk.

Britain Before Brexit: Northern Ireland Show all 12 1 /12 Britain Before Brexit: Northern Ireland Britain Before Brexit: Northern Ireland Derry, Londonderry A garage door displaying unionism, bolted shut, like a visual representation of Brexit Britain, locked to outsiders, safeguarding what’s inside Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Derry, Londonderry Rossville Street, the site of Bloody Sunday, where messages demand a severance with England. From this perspective, Britain is England in sheep’s clothing, the real empire, the centre of colonial power Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Bangor A political message in paint not yet dry, still forming, setting, adjusting, or in old paint finally eroding, melting away Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Bangor Moral judgement frames a residential view. The message seeks to make everybody involved in the religious narrative: those who don’t believe are those most in debt Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Castlerock The beach is sparse and almost empty, but covered in footprints. The shower is designed to wash off sand, and a mysterious border cuts a divide through the same sand Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Belfast Two attempts to affect and care for the body. One stimulated by vanity and social norms and narratives of beauty, the other by a need to keep warm in the winter night Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Belfast The gate to an unclaimed piece of land, where nothing is being built, where no project is in the making, where a sign demands the creation of something new Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Derry, Londonderry Under a motorway bridge a woman’s face stares, auburn and red-lipped, her skin tattooed with support for the IRA and a message of hostility to advocates of the Social Investment Fund Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Derry, Londonderry The Fountain Murals, where the curbs and the lampposts are painted the red, white, and blue of the Union Flag. A boy walks past in the same colours, fitting the scene, camouflaged Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Coleraine A public slandering by the football fields, for all to see or ignore. I wonder if it’s for the police or for the community Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Belfast A tattoo parlour, where the artist has downed tools, momentarily, bringing poise to the scene, which looks like a place of mourning, not a site of creation Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Derry, Londonderry A barrier of grey protects the contents of this shop, guarding it from the streets outside, but it cannot conceal it completely, and the colours of lust and desire and temptation cut through Richard Morgan/The Independent

But, in an article for the Conservative Home website, Mr Howe argued it was worth even the gamble that delay would allow opponents to “reverse Brexit altogether”.

Crucially, extending Article 50 until January 2021 – a date mooted in Brussels – would prevent the UK having to adopt the backstop and becoming trapped, “unlike the deal,” he said.

It would also cut the estimated £39bn ‘divorce bill’ and avoid “indefinite ECJ [European Court of Justice] jurisdiction”, while retaining the UK’s “vote and voice within the EU institutions”.

“The problem with May’s deal is that it poisons Brexit by closing off the freedom of action which is the whole point of Brexit, and drains away its advantages,” Mr Howe wrote.

“If Brexit supporters are complicit in miring the UK for a decade or a generation in such a terrible vassal arrangement with the EU, inevitably calls will grow for us to re-join the EU in order at least to get back a vote on the rules which we have to follow.

“On a positive note, even if the Remainers were to get the long extension which they crave, it does not mean that the support will be there either in parliament or still less in the country to hold a second referendum or to reverse Brexit.

“The Labour party in particular would find it very difficult if it alienates its Leave-supporting voters in the many marginal seats where they are plentiful.”

Mr Howe predicted that the prime minister’s threat of her deal or a long delay – foreshadowed in a leak of her Brexit adviser’s private comments last month – would backfire.

“The ‘threat’ is manifestly more advantageous in every way than the thing the threatener wants the threatened to do (vote for the May deal),” he wrote.

The article came after Downing Street appeared to water down Ms May’s demands from the EU, refusing to say the UK is still fighting for a “time limit” to the backstop, or an “exit clause”.

The prime minister is pinning her hopes of passing the deal on Geoffrey Cox, the attorney general, changing his legal advice that the UK is in danger of being trapped in the backstop indefinitely.