The Democratic Unionist party is to seek an urgent meeting with Boris Johnson to warn him it will stall power-sharing at Stormont until Christmas if the Northern Ireland protocol is not modified.

A senior party figure said it will push the responsibility for Stormont back on to the prime minister: “We will say he has a choice: Stormont or the protocol.”

The ultimatum came as the UK justice secretary, Dominic Raab, pledged to do “whatever necessary” to alter the protocol, which mandates checks on goods crossing into Northern Ireland from Great Britain.

But the DUP’s trust in the Conservative party, which was already at a low ebb, was shattered last week when the Northern Ireland secretary, Brandon Lewis, hinted the government was pulling back from threats to introduce laws to unilaterally disapply parts of the protocol.

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The party’s leader, Sir Jeffrey Donaldson, will lay out his concerns in a statement on Tuesday following the Queen’s speech in the House of Commons.

The Queen’s speech is expected to include a commitment to protect the interests of the unionist community and the Good Friday agreement, but the DUP says it will not go as far as it wants.

The re-elected DUP MLA for Upper Bann, Jonathan Buckley, told the Belfast Telegraph on Sunday that Lewis also needed to take responsibility for changing the protocol: “Either the secretary of state wants an executive or the protocol, he can’t have both .”

Lewis has said he will meet all the Northern Ireland party leaders in the coming days but the DUP will stick to its position that it has support for a boycott of Stormont and only an intervention by London can restore power-sharing. “We can stall this until Christmas,” said one senior DUP insider.

The party argues that despite the surge in support for Sinn Féin in last week’s election, the majority of Stormont seats (35) have gone to unionist parties – 25 to the DUP, nine to the UUP and one to the Traditional Unionist Voice party.

Doug Beattie, the Ulster Unionist leader, said he had been invited to meet Lewis on Monday and expected the Stormont assembly to resume work this week regardless of the formation of an executive. “I expect all our MLAs to be there. We have a lot to be getting on with.”

Claire Sugden, an independent unionist who held her seat in East Derry, said she favored forming an executive as soon as possible. “I appreciate there are issues with the protocol but I want to see an executive now, up and running.” The protocol was not a constitutional issue but a logistical one that required fixing, she said.

Sugden said she would not join any party at Stormont, adding: “The people elected me as an independent.” There has been speculation the DUP might try to co-opt independent unionist MLAs to overtake Sinn Féin’s numerical superiority.

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One insider in Belfast said the party required substantial reform of the protocol, with one of their MPs warning they needed to see all the European court of justice’s role extinguished – which was previously a sticking point in negotiations with the EU. “All the energy in unionism right now is on the right and is demanding serious changes.”

DUP insiders say they will press home to the prime minister that new laws designed to prevent a future collapse of the assembly also give it the power to delay formation of the executive for up to six months.

Under legislation passed in February the parties have six weeks following the opening of the assembly to form an executive. If they fail to do so they can be given three further six-week extensions. If there is no executive by the end of the 24 weeks, then the Northern Ireland secretary must call an election, which must be held within 12 weeks.

The deputy leader of the centrist Alliance party, Stephen Farry, who more than doubled his seats in the assembly election, could prove an ally to the DUP.

Farry said at the weekend it had to be recognized more widely that the protocol needed to be changed, but he called for a “pragmatic approach” with all parties working together in Stormont rather than another showdown with Brussels.

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