The top Tories who cited NI in their resignation letters to Theresa May

In all, four ministers published letters accusing the Prime Minister of having come up short in the Brexit negotiations.

They were Brexit secretary Dominic Raab, his junior Suella Braverman, junior minister in the Northern Ireland Office Shailesh Vara, and work and pensions secretary Esther McVey.

In addition to these ministers, three other MPs also stood down from positions they were holding:

Anne-Marie Trevelyan resigned as a parliamentary private secretary in the Department for Education, Ranil Jayawardena also quit his post as a parliamentary private secretary in the Ministry of Justice, and Rehman Chishti resigned as Conservative vice-chairman and prime ministerial trade envoy to Pakistan.

Of the seven MPs in total, five made specific reference to the Union as reasons for quitting.

Mr Raab’s letter said: “I believe that the regulatory regime proposed for Northern Ireland presents a very real threat to the integrity of the UK.”

Esther McVey said the deal “threatens the integrity of the United Kingdom, which as a Unionist is a risk I cannot be party to”.

Mr Vara said Northern Ireland “will be subject to a different relationship with the EU from the rest of the UK and whilst I agree there should be no hard border between Northern Ireland and Ireland, the economic and constitutional integrity of the United Kingdom must be respected”.

Mrs Braverman said “the backstop proposals set out different regulatory regimes for Northern Ireland and Great Britain threatening to break up our precious Union”.

She added: “I am confident – having met with customs professionals in my role at the Department – that this could have been avoided.”

And Anne-Marie Trevelyan had this to say in her resignation letter: “As an MP bordering on Scotland, the regulatory framework agreement for Northern Ireland is very important to me, and I cannot support the position the EU agreement takes. I believe that it poses a real threat to the stablity and integrity of the Union.”