Arlene Foster’s trip to Brussels is a stark warning for Theresa May

Foster’s meeting with Barnier reminds the government that a regulatory border down the Irish Sea is a surefire way to lose the DUP’s backing.

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Guess who’s coming to dinner? Arlene Foster, the DUP’s leader, is in Brussels for the next three days to talk about her party’s priorities for Brexit and to underline that her only red line is that Brexit does not result in the creation of additional regulatory barriers between Northern Ireland and the rest of the United Kingdom.

The opening preamble of the DUP’s 2017 manifesto – which warned unionist voters that whichever party got the most seats in Northern Ireland would be seen as that part of the world's true voice – has never felt quite so prophetic. It's partly about meeting with Barnier as the various other party leaders have done but also about sending a message to the Conservative Party.

It’s not uncommon to hear Tory MPs say that, look, there is a regulatory border for cows – would a regulatory border for electricals really spell the end of the Union? Perhaps that’s a price worth paying for a Canada-style Brexit. Don’t forget that for all Theresa May likes to chunter on about how “no British Prime Minister would accept” a regulatory border down the Irish Sea, that was the deal she was all dressed up and ready to accept over cinnamon-scented ice cream before the DUP made it clear they would never accept such an arrangement.

What Foster is saying loud and clear with her meeting with Barnier is if the Conservative Party decides that a regulatory border down the Irish Sea is a price they are willing to pay in Brussels, that cheque will bounce and so will they.

Stephen Bush is political editor of the New Statesman. His daily briefing, Morning Call, provides a quick and essential guide to domestic and global politics.