Theresa May will on Thursday ask her Brexit “war Cabinet” to agree a backstop plan that would keep Britain in a customs union with Brussels until a permanent trade deal can be agreed.

British and EU negotiators are understood to have agreed in principle to an all-UK backstop plan to avoid a hard border in Northern Ireland that would remove the final major obstacle blocking a withdrawal agreement.

Boris Johnson said the deal would turn the UK into a “permanent EU colony” and the DUP angrily threatened to break its confidence and supply deal with the Conservatives and potentially bring down the Government if the Prime Minister goes through with the plan, which it described as a “sell out”.

On Wednesday night the DUP showed it was serious by abstaining from a vote on a Labour amendment to the Agriculture Bill after its MPs stated their demands during a meeting with Julian Smith, the Chief Whip.

The backstop plan being proposed by Mrs May would involve the whole of the UK remaining in a customs union with the EU while negotiations over a free trade deal take place, which Brexiteers fear could take years.