Brexit Party leader Nigel Farage has said that the deal agreed between Boris Johnson and the EU is “just not Brexit”, while the DUP has said it will not back it in parliament.

The prime minister and EU bureaucrats agreed on a Brexit deal on Thursday morning which eliminates the Irish backstop and puts in place other measures to ensure EU customs regulations are respected on the isle of Ireland whilst keeping Northern Ireland in the UK’s customs territory.

Negotiations came down to the wire, concluding on the opening day of the European summit, at which the European Commission President Jean-Claude Juncker said the EU27 should back the deal. If supported by European leaders, the agreement will go to the House of Commons for a vote on Saturday.

While the Republic of Ireland is satisfied with the arrangments, the conservative, unionist DUP has said its MPs will not vote for the deal in parliament. The party said in a statement this afternoon: “Following confirmation that the prime minister now believes that he has secured a ‘great new deal’ with the European Union, the Democratic Unionist Party is unable to support these proposals in parliament.”

The leftist Scottish Nationalist Party (SNP), Labour, and the Liberal Democrats — which all have in their party manifestoes strong, anti-Brexit stances — have said they will not back the deal on Saturday.

Brexit Party leader Nigel Farage told the BBC that “it’s just not Brexit”.

“We’ve taken three-and-a-half years to get to this point. If this was to be agreed, we then enter into years more negotiations for the prize of a free trade agreement which we already know we will not get unless we surrender our territorial fishing waters and will not get unless we stay in regulatory alignment with the European Union.”

Mr Farage added that the deal would mean the UK surrendering autonomy on making her own laws on employment regulations and the environment, as well as signing up the country to foreign policy and military policy commitments.

The Brexit Party leader said that he would “prefer to have an extension” — another Brexit delay — “and a general election” than “accept a new European treaty”.

“I would genuinely believe that a clean break and being able to be competitive is the absolute key to our future economic success. We cannot do that with this new treaty. We will never be able to properly break free of the EU if we sign up to this,” he concluded.

The Conservative Party’s European Research Group (ERG) has been rallying around the prime minister in recent weeks over his efforts to negotiate a new deal, even while the hardcore “Spartans” remain in favour of a clean-break Brexit. However, no official statement has come from its chairman, Steve Baker, or deputy, Mark Francois, on their position on the deal to date.

However, ERG member Nigel Dodds implied that some members of the ERG had changed position and were open to backing a new deal.

Speaking to BBC Radio 4’s Today programme on Thursday morning, Mr Dodds said that “some of the mood music now coming from people like Steven Baker is completely different to where we were just a couple of months ago”.

“You can’t underestimate the colossal achievement of where we now are,” the ERG member said praising the deal, adding: “Who would have thought that from when I last chatted to you a few days ago we would now be on the cusp of an agreement?”

While last night, Mr Francois hinted that the ERG would not automatically vote the same way as the DUP. Telling Sky News that the ERG is not a “Stalinist group” and members were free to “look into their conscience” to decide how to vote, he said: ”The ERG and the DUP have always been firm allies all the way through this process.

“It’s not axiomatic that we would automatically vote the same way as them, but particularly as these arrangements have strong implications for Northern Ireland, we would give very strong weight to whatever the DUP say.”

Former Conservative Party leader and Brexiteer Iain Duncan Smith said, “I’m reserving my position” on whether or not to vote for the deal “because I really want to read what’s in it”.

“I want to know how quickly can we get to a free trade agreement, how quickly can we, therefore, get out, what incentive is there for the EU to do this quickly? And is the political declaration exclusively for free trade?” he said.

Leader of the House of Commons and former ERG chief Jacob Rees-Mogg threw his support behind the deal, urging MPs to vote for it.