Brexit deal: Details of Boris Johnson's agreement reveal why furious DUP is set to defeat it

No 10 insisted it would not repeat Theresa May’s fatal mistake of signing an agreement without DUP support – but now it has

The details of the deal struck in Brussels lay bare why the Democratic Unionist Party believes it has been betrayed – and is likely to sink it at Westminster.

In a stinging statement, Arlene Foster’s party accused Boris Johnson of undermining the union and the “professed sanctity” of the Good Friday Agreement.

The Tories’ Northern Ireland allies protesting about the agreement in three areas – customs, consent and VAT.

* Consent – the most important objection

The DUP signed up to the prime minister’s proposals last week because they promised a Unionist veto at Stormont, allowing it to block – or later remove – alignment with EU customs and single market rules.

The new deal removes that veto, exercised by just 30 votes in the 90-member Northern Ireland Assembly, and replaces it with consent for barriers in the Irish Sea with just a simple majority.

Furthermore, the vote would only take place four years after the end of the transition period, scheduled for the end of 2020 – not before alignment with EU rules is introduced.

One Unionist fear is that Republicans could simply collapse the assembly – which has not sat for nearly three years, because of cross-community disputes – trapping them in what is effectively a Northern Ireland-only backstop.

* Customs

The big win for the prime minister – and the DUP – is that Northern Ireland will be within the UK’s customs territory and “benefit” from new trade deals struck with third countries.

However, the agreement states that “goods travelling from Great Britain to Northern Ireland will be subject to customs duties if there is a risk of them being subsequently moved south of the border”.

So, there will be checks on all goods at a new border in the Irish Sea – once a red line for Mr Johnson himself, who told the DUP conference last year that “no British Conservative government could or should sign up”.

* VAT

The change is that it moves Northern Ireland closer to adopting Irish VAT rates – even though the UK would collect and retain revenues.

The DUP fears this will lead to increasing convergence with Dublin “without broad support” in the North.

Unsurprisingly, the DUP concluded: “These proposals are not, in our view, beneficial to the economic wellbeing of Northern Ireland and they undermine the integrity of the union.”