A government minister has quit his job protesting that Theresa May has risked the UK’s “final humiliation” by opening the door to delaying Brexit.

George Eustice resigned as farming and fisheries minister, warning: “Developments this week will lead to a sequence of events culminating in the EU dictating the terms of any extension requested and the final humiliation of our country.”

In a letter, Mr Eustice – a supporter of the prime minister’s stalled deal – attacked her for preparing the ground for taking a no-deal Brexit off the table.

“If the position of parliament is now that we will refuse to leave without an agreement then we are somewhat stuck,” he wrote.

“This is uncomfortable for everyone, but we cannot negotiate a successful Brexit unless we are prepared to walk through the door.”

The walkout comes after Ms May bowed to growing cabinet pressure by agreeing to grant MPs a vote on extending Article 50 and delay departure beyond 29 March.

The vote will take place on 14 March if the Commons first rejects the deal on the table, in a second “meaningful vote”.

A former Ukip candidate, Mr Eustice told the prime minister he would still vote for her withdrawal agreement and that he hoped attempts to win changes to the Irish border backstop would succeed.

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

He said she had been “terribly undermined by those in parliament who refuse to respect the referendum result” and paid tribute to her “tenacity and resilience over the past year”.

But he wrote: “What our country needs from all its political leaders at this critical juncture is courage, and we are about to find out whether parliament has it.”

The Camborne and Redruth MP’s continued backing for the deal – thrown out in a record 230-vote mauling six weeks ago – makes his resignation different to the others that have scarred Ms May’s premiership.

Instead, his protest is at the growing likelihood of delay, unless scores of Brexiteer Tories can be persuaded to change their votes on 12 March.

It underlines that some May supporters will, given the choice, prefer to crash out of the EU on schedule on 29 March rather than accept a delay.

The fear is that, following Emmanuel Macron’s comments this week, the EU will exploit the hold-up to forge a softer Brexit, or no Brexit at all.

Mr Eustice also attacked the European Commission for failing to “behave honourably during these negotiations”.