John McDonnell has played down speculation that any more than a handful of Labour MPs could vote for Boris Johnson’s Brexit deal on Saturday, saying he was hoping to persuade his colleagues to hold firm.

Several have already said they will back the plan. McDonnell, the shadow chancellor, said there would be “consequences” for any who did, but did not specify whether this could include losing the party whip.

With the Democratic Unionist party holding firm in its opposition to an arrangement that creates an effective customs border in the Irish Sea, the votes of rebel Labour MPs could be crucial in Saturday’s vote.

McDonnell told BBC Radio 4’s Today programme that he expected the numbers to be limited. He said: “I don’t believe there are Labour MPs out there who will not think through the detail of this deal and then when they see the impact, particularly environmental, consumers’ and workers’ rights, will vote for it.”

Among Labour MPs who have said they will vote for the deal are Jim Fitzpatrick, who represents Poplar and Limehouse, and the Blyth Valley MP, Ronnie Campbell.

Quick Guide How is Boris Johnson's Brexit deal different from Theresa May's? Show The new Brexit deal is essentially the old Brexit deal with a new chapter on the protocol on Ireland and Northern Ireland and a few key tweaks to the political declaration. Here is a link to the full text. The backstop is replaced The backstop has essentially been replaced by a full stop whereby Northern Ireland remains aligned to the EU from the end of the transition period for at least four years. A change can only happen if it is voted on by the Stormont assembly. Consent Stormont will have a key role in future Brexit arrangements. And if there is cross-community support to remain aligned to the EU rather than the UK the consent will hold for eight years. The arrangements in this deal will automatically kick in for a mandated four years if there is a breakdown in trade talks, so it remains a “backstop” but with a permanent tinge. That four-year period will start at the end of December 2020. Two months before the end of the four-year period, that is October 2024, Stormont will be asked to vote on whether to remain aligned to the EU in ways outlined by this deal or not. Checks on border, ports and airports Under the deal, the UK and the EU are “underlining their firm commitment to no customs and regulatory checks or controls and related physical infrastructure at the border between Ireland and Northern Ireland”. Future trade deals The EU and the UK will aim for a zero-tariff deal with unlimited quotas. The entire UK, including Northern Ireland, will be free to sign trade deals. The line in the political declaration that “the United Kingdom will consider aligning with union rules in relevant areas” in any future trade talks has been ditched. Customs Northern Ireland will remain legally in the UK customs territory but practically in the EU customs unions. There will therefore be no customs checks on the border but tariffs will be payable on certain commercial goods. No customs duties will be payable on “personal property” being transited from Great Britain to Northern Ireland. That protects online shopping and all items bought for personal rather than commercial use. Customs duties will be payable on goods imported from the UK for commercial use unless it can be demonstrated that the goods remain in Northern Ireland or are for personal use, as above. A system of rebates will allow importers to be reimbursed. West/east trade The commitment to frictionless trade between Northern Ireland and Great Britain is restated. VAT EU law on VAT will apply in Northern Ireland. Single electricity market The island of Ireland is considering a single market for electricity so homes in Northern Ireland can get their energy from a supplier in Northern Ireland or the republic. There were fears this could be disrupted by Brexit. Under the Johnson deal, the provisions of union law remain so nothing will change. Level playing field This guarantees that the UK will remain in line with EU conventions on climate, environment and workers rights in a future trade agreement. Lisa O'Carroll Brexit correspondent

Asked about Campbell’s views, McDonnell said: “I’m going to have a chat with Ronnie because he is a good socialist and all his life he has fought alongside me to protect trade union rights and develop trade union rights.

“On this one I’m going to have a chat with him and point out to him: please do not give this power to Boris Johnson because you know what he’ll do. He’ll undermine trade union rights. Boris Johnson and those Tory MPs that populate the cabinet, these are the extreme right who have attacked trade unions throughout their political careers.”

He added: “No MP, as far as I’m concerned, who has the true interest of their constituents at heart can allow that to happen.”

Asked what would happen to Labour MPs who supported the deal, McDonnell would not say whether they could lose the whip.

“This is a three-line whip and the chief whip, in the normal way, will determine the consequences for anyone who doesn’t vote for it,” he said. Asked what they might be, he replied: “I’m not the chief whip thank goodness; I’ve got enough on my plate.”

Earlier, the DUP’s Brexit spokesman, Sammy Wilson, told Today that he could give his “absolute assurance” the party would not back the deal.

On Johnson’s concessions to the EU over different customs arrangements for Northern Ireland, Wilson said: “We are disappointed he didn’t stick to the red lines he said he would.”

Wilson said he believed a better path would be for the deal to fall and Johnson to seek a bigger Commons majority via an election: “I believe, with a big majority, he can be more robust in his negotiations,” he said.

“It is one of the reasons why we believe that voting this down tomorrow is not the end of the game but in fact probably opens up possibilities for the government that are not available at present but will be after a general election.”