Foreign Secretary Liz Truss says she will not accept a deal which means goods from Britain being checked as they enter Northern Ireland.
Ms Truss, now the UK’s lead negotiator with the EU in post-Brexit talks, was writing in the Sunday Telegraph.
In 2019 the prime minister agreed a deal, known as the protocol, and for the last six months the UK has been attempting to renegotiate it.
The EU said it was “not too impressed” with the foreign secretary’s threat.
Next week, Ms Truss is due to hold two days of talks with her European Union counterpart, Maroš Šefčovič.
“I will not sign up to anything which sees the people of Northern Ireland unable to benefit from the same decisions on taxation and spending as the rest of the UK, or which still sees goods moving within our own country being subject to checks,” she said.
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