FILE PHOTO: Brandon Lewis arrives in Downing Street, London, Britain, December 18, 2018. REUTERS/Hannah McKay

LONDON (Reuters) - Britain cannot accept an EU proposal to break the deadlock in the Brexit talks because it would threaten the unity of the United Kingdom by treating Northern Ireland differently, the chairman of the ruling Conservative Party said.

The issue of how to maintain an open border on the island of Ireland is at the heart of the dispute between London and Brussels after British lawmakers objected to the so-called backstop insurance policy which they believe would keep the whole of the UK trapped in an EU customs union.

Less than three weeks before Britain leaves the European Union, the EU’s chief negotiator Michel Barnier said on Friday Britain could have the unilateral right to leave their customs union after Brexit.

However, Northern Ireland would have to remain in the EU’s trade orbit to prevent the need for any customs checks on the border with EU-member Ireland.

“We are not going to have an agreement that compromises the unity of the United Kingdom,” Brandon Lewis told BBC radio. “The proposal that Michel Barnier put out yesterday would compromise the strength of the union.”

Lawmakers will vote again on Tuesday on whether to accept the deal struck by Prime Minister Theresa May. The government has so far failed to secure changes to the divorce deal that could gain the backing of lawmakers, after they roundly rejected it in January.

Lewis said talks would continue over the weekend.