The Democratic Unionist party is reeling from accusations it was duped and betrayed by Downing Street over a Brexit deal that weakens Northern Ireland’s position in the UK.

Rival unionists on Thursday said the DUP had committed a catastrophic miscalculation in trusting Boris Johnson, potentially putting Northern Ireland on a path towards a united Ireland.

The DUP also risked isolation at Westminster where some Conservative Brexiters, who previously pledged solidarity with the DUP, signalled they would ignore the party’s opposition and vote for the deal in a Commons vote on Saturday.

It was a vertiginous fall to earth for a regional party that until recently boasted of unprecedented influence, its clout extending from Belfast to London and Brussels.

Robin Swann, leader of the Ulster Unionist party (UUP), said DUP blunders had paved the way to Northern Ireland being left on the “window ledge” of the union.

“It’s an absolute disgrace ... this deal is worse than the one Theresa May brought forward at Chequers. It’s awful. It would put a customs and regulatory border down the Irish Sea and annexes Northern Ireland from the rest of the United Kingdom.”

Jim Allister, leader of the Traditional Unionist Voice party, called the deal – welcomed by the Irish government and Sinn Féin – a disaster that would put Northern Ireland in the Republic of Ireland’s economic orbit, ensuring the union “slowly bleeds to death”.

“The inescapable reality is that a permanent regulatory and customs border cutting us off from GB puts us in a waiting room for Irish unity with the door locked from the outside.”

Many farmers and business owners, in contrast, welcomed a deal that would keep trade flowing and avoid a hard border on the island of Ireland. “What is good is that a deal is struck, we can move on to the more positive conversation about the future relationship and, regardless, Northern Ireland can continue to have tariff-free, quota-free access to both the UK and the EU’s single market,” said the Manufacturing Northern Ireland chief executive, Stephen Kelly.

They are relieved they have certainty as the new arrangements will mean the same trading environment for four years beyond the transition period. Stormont then gets to vote, and if the new regime is popular and can command 60% support – with at least 40% each of the nationalist and unionist votes – it will be rolled over for another eight years, a protracted EU alignment that is anathema to the DUP.

The party’s deputy leader, Nigel Dodds, told the BBC that he felt the Benn act preventing no deal had pushed Boris Johnson “into desperate measures”.

Dodds said: “He has been too eager by far to get a deal at any cost, and the fact of the matter is, if he held his nerve and held out he would, of course, have got better concessions that kept the integrity, both economic and constitutionally, of the United Kingdom.”

Under Johnson’s deal, Northern Ireland would legally remain in the UK’s customs territory but stay in the EU’s single market for goods. The EU’s customs code would be enforced on goods coming from Great Britain into Northern Ireland.

Under a complicated consent mechanism, four years after the end of the transition period, the Northern Ireland assembly at Stormont would vote on maintaining the arrangements. Crucially, the DUP would have no veto.

The DUP rejected the deal and vowed to vote against it, saying it would undermine the integrity of the union and drive “a coach and horses” through the Good Friday agreement’s consent mechanism.

Once the details became clear DUP leaders pivoted towards a campaign to retain their base, said one source. “Shouting traitor from the green benches might suit them.”

Some Brexiters will blame the party if the deal falters in the Commons, further straining relations between Northern Ireland’s unionists and English Conservatives.

Rival unionists said DUP hubris led to the deal.

A year ago the party’s leader, Arlene Foster, boasted of its unprecedented influence at Westminster – where its 10 MPs propped up the government – and garlanded Johnson at its party conference. Johnson said Northern Ireland should be treated no differently from the rest of the UK.

Two weeks ago Johnson, under pressure for a deal with Brussels, persuaded his allies to accept some regulatory alignment with the EU, necessitating checks in the Irish Sea.

That concession – a DUP recognition that farmers and business owners feared a no-deal hit on Northern Ireland’s economy – facilitated Downing Street stretching the measure to clinch a deal with Brussels, said Jon Tonge, a University of Liverpool politics professor and co-author of a book about the DUP.

“They’ve been completely stitched up by Boris Johnson. He reversed on all that he promised them. This deal gives the EU and Irish government huge economic leverage over Northern Ireland.” Tonge called the consent mechanism a fantasy given Stormont’s dysfunction – it has been mothballed since power sharing collapsed in January 2017.

The anti-abortion DUP may receive another humiliation on Monday when abortion rights are due to be extended to Northern Ireland, the result of a vote by Labour and Conservative MPs at Westminster earlier this year.

The Stormont assembly is due to sit on Monday for the first time since devolution collapsed after more than 30 members backed a recall petition to debate the law. Analysts said it was unlikely they could stop the law taking effect.