(Bloomberg) -- Northern Ireland’s Democratic Unionist Party and a handful of Conservative Members of Parliament issued a warning to UK Prime Minister Rishi Sunak over Brexit, foreshadowing domestic obstacles ahead as his government seeks to end a dispute with the European Union.

The DUP’s 8 MPs plus six Tories called on ministers to abandon regulations enshrining plans to build new border control facilities in Northern Ireland, as required by the UK’s existing deal with the EU. The Conservative signatories include former ministers Jacob Rees Mogg, John Redwood and David Jones, influential figures on the party’s pro-Brexit wing.

“The regulations give effect to a customs border that divides the United Kingdom, treating Northern Ireland like a foreign country,” the MPs wrote in a non-binding so-called early day motion proposed in Parliament this week. “It is their purpose to protect the integrity of a different legal regime in Northern Ireland created by laws in over 300 areas imposed by a polity of which it is not a part and in which it has no representation.”

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The motion itself will be of little concern to Sunak: EDMs are typically used by MPs to register opinions or dissent and rarely get formally debated. But it’s also a reminder of the caucus of lawmakers he needs to satisfy as British negotiators home in on a new deal with the EU to resolve the thorny issue of Northern Ireland’s post-Brexit trading arrangements.

The government is trying to re-negotiate the so-called Northern Ireland Protocol, the part of the Brexit agreement that effectively keeps that region in the EU’s single market, and establishes a customs border between it and Great Britain. 

With the UK and EU nearing a provisional agreement, Sunak will be wary of the coordination between the DUP and Brexiteer Tories in the pro-Brexit European Research Group. That’s because the ERG has made life difficult for successive prime ministers, while the DUP has blocked the formation of Northern Ireland’s regional government in protest against the current deal — and could similarly stymie a new agreement.

The DUP has set seven tests for a deal, while a prime ERG concern is that there should be no role for the European Court of Justice. They may be disappointed in that demand, after Sunak’s spokesman suggested on Thursday that the UK may be willing to allow the ECJ to retain a role.

“The UK and EU can talk until the cows come home,” said Jim Shannon, a DUP MP, speaking in a phone interview. “That agreement holds no water if it doesn’t address the issue of unionism.”

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