The Democratic Unionist Party (DUP) has voted for a proposal that could delay Brexit until all necessary UK laws are passed in Parliament.
MPs voted by 322 to 306 to pass the so-called Letwin amendment to the government’s Brexit deal, inflicting a blow to the prime minster’s proposals.
The DUP backs Brexit, but does not support the prime minister’s revised proposals for Northern Ireland.
It is not clear when Number 10 will now hold a meaningful vote on its deal.
MPs are holding a rare Saturday sitting to debate the government’s Brexit deal, and earlier the DUP’s deputy leader said his party would vote against it.
Boris Johnson will now try to get his deal across the line without the support of the DUP’s 10 MPs.
Mr Dodds urged the prime minister to reconsider the proposals, which include special terms for Northern Ireland.
The DUP is opposed to the consent mechanism in the Brexit deal, which would give the Northern Ireland Assembly a say on whether to continue following EU customs rules.
It would take place by a simply majority vote: pro-EU parties have a narrow majority at Stormont and there would be no unionist veto, as demanded by the DUP.
Veto demand a ‘pity’
Mr Dodds said Boris Johnson needed to respect the concerns of unionists – but the prime minister dismissed suggestions that his deal breached the principle of consent.
“In all frankness I do think it a pity that it is thought necessary for one side or the other of the debate in Northern Ireland to have a veto on those arrangements,” he told MPs.
He argued that the Brexit referendum had taken place on a straight majority basis, adding: “I think that principle should be applied elsewhere, I see no reason why it should not apply in Northern Ireland as well.”
On Saturday morning, DUP MP Sammy Wilson met with members of the Eurosceptic European Research Group (ERG), but it has encouraged its members to support the deal.
Independent unionist MP for North Down, Lady Hermon, has not confirmed whether she will support the government’s plan.
She told Mr Johnson there is “anger” in Northern Ireland’s unionist community over his deal – but the PM said he is committed to the constitutional position of Northern Ireland, calling it “inviolable”.
Before the vote on the deal can go ahead, MPs are voting on an amendment – tabled by Oliver Letwin – that would withhold approval of the deal until the legislation to enact it was safely passed – a move that would automatically trigger the “Benn Act” and force the prime minister to request a further postponement of Brexit until 31 January.
A DUP source told the BBC the party would vote for the amendment, representing a blow to the government’s strategy.
Downing Street has threatened to postpone the vote on the revised deal altogether if the amendment is passed on Saturday.
What does the deal involve for NI?
The new Brexit deal would involve Stormont giving ongoing consent to any special arrangements for Northern Ireland via a straight majority, instead of on a cross-community basis.
Northern Ireland would continue to follow EU rules on food safety and product standards and would also leave the EU customs union.
But EU customs procedures would still apply on goods coming into Northern Ireland from Great Britain in order to avoid checks at the border.
Stormont would have to approve those arrangements on an ongoing basis.
Approval would involve a straight-forward majority, which would keep the special arrangements in place for four years.
Alternatively, if the arrangements are approved by a majority of nationalists and a majority of unionists, they would remain in place for eight years.
If the Northern Ireland Assembly voted to end the arrangements there would be a two-year notice period, during which the UK and the EU would have to agree ways to protect the peace process and avoid a hard border.
If a vote was not held – by choice or because the assembly was not sitting – then the government has committed to finding an “alternative process”.
Ex-PMs unite to oppose deal
Former prime ministers Sir John Major and Tony Blair appealed on Friday for MPs not to vote for the deal, claiming it would “wreck” the 1998 Good Friday peace agreement, which led to the end of the Troubles in Northern Ireland.
They called for another referendum on Brexit.
Former Northern Ireland first minister Lord Trimble, who won a Nobel Peace Prize for his role in Good Friday Agreement, backed the deal.
In a statement published by the Spectator, the former Ulster Unionist leader said it was a “great step forward” that was “fully in accordance with the spirit of the Good Friday Agreement”.
“What we now want to see is for the DUP and Sinn Féin to act together to bring the Good Friday Agreement back to life,” said the Tory peer.
“This is not the time to be looking for excuses not to implement either the Good Friday Agreement or the new deal.”