Boris Johnson‘s hopes of securing a Brexit deal have been dealt a blow after the DUP‘s deputy leader warned that the plan reportedly being considered by UK and EU officials “cannot work”.

Nigel Dodds, who also serves as the party’s Westminster leader, dismissed any suggestion that Northern Ireland could in effect remain in the EU’s customs union when the rest of the UK leaves.

Such an outcome is reported to be a key part of the plan being discussed during intense talks in Brussels over the weekend.

It emerged following talks between Mr Johnson and his Irish counterpart, Leo Varadkar, on Thursday, after which the EU agreed to intensify negotiations.

Mr Johnson is reported to have made significant concessions on the issue of customs after Brexit, most likely by resurrecting Theresa May’s proposal for a “customs partnership” between the UK and EU as an alternative to the controversial Northern Ireland backstop.

This would involve Northern Ireland applying the same customs as the EU, thus removing the need for customs checks on the Irish border, but with the collection of tariffs being managed by the UK. Northern Ireland would also be part of any trade deals the UK signed with other countries.

Mr Johnson is almost certain to need the DUP’s ten votes in the House of Commons to get any deal through parliament, and a number of Tory Eurosceptics are likely to take their lead from the Northern Irish party.

But Mr Dodds told Italian newspaper La Repubblica: “Northern Ireland must stay in a full UK customs union, full stop.

“There is a lot of stuff coming from Brussels, pushed by the Europeans in the last hours, but one thing is sure: Northern Ireland must remain fully part of the UK customs union. And Boris Johnson knows it very well.”

He added: “It cannot work because Northern Ireland has to remain fully part of the UK customs union.”

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Mr Dodds also said a tweet posted on Friday by Tory Brexiteer Owen Paterson was “absolutely right”.

The tweet read: “Northern Ireland being held in the customs union without Great Britain is an obvious breach of the principle of consent in the Belfast Agreement. This would stir up problems in Northern Ireland long after Brexit is resolved.”