Boris Johnson has accused Remainers of using the Irish border issue to try to "frustrate" Brexit as a fresh row brews over the terms of the UK's withdrawal.

The prominent Brexiteer said that concerns over a potential hard border were being used as an excuse to keep Britain "effectively in the single market, so we can't really leave the EU".

It comes as Brussels prepares to publish a draft withdrawal treaty which is said to include proposals for Northern Ireland to remain aligned with Brussels if no other solution to avoid a physical barrier can be found.

Theresa May has insisted that a solution can be reached on the issue.

But concerns were reignited after Mr Johnson sent a letter to the Prime Minister saying it was wrong to suggest there would be no border and the focus should be on ensuring it does not become "significantly" harder.

Today he insisted the Irish issue was being "used politically" as a tool to keep the UK within the customs union.

"I think the particular problems around the Irish border are being used to drive the whole Brexit argument and effectively to try to frustrate Brexit," he told reporters.

His comments come as Mrs May faces pressure to reveal more details on her plans for Brexit as Brussels gives more details on its vision.

Brussels' chief negotiator, Michel Barnier, said there would be "no surprises" in the draft withdrawal treaty text, which is based on agreements made between Mrs May and Commission president Jean-Claude Juncker in December.

Any suggestion that Ireland could diverge from Great Britain will be a red line for the DUP party which is propping up Mrs May's government.

The text is also said to propose the UK remains under the jurisdiction of European Court of Justice for any treaty-related disputes - a plan that is sure to outrage hard Brexiteers in her cabinet such as Mr Johnson.

Former Brexit minister David Jones said the EU's proposal was "completely unacceptable" and amounted to an attempt to annex Northern Ireland, in comments to BBC Radio 4's Today programme.

"I think that it would be pretty catastrophic and I think that the European Union in actually proposing this isbehaving wholly irresponsibly."

Downing Street was swift to play down Mr Johnson's comments in his leaked letter.

A source said: "We agree the task isn't about no border, it's about no hard border."

Mrs May is due to set out the UK's approach to the next phase of talks in an announcement on Friday.

She is also facing fresh pressure from backs of a soft Brexit after the Labour party said it backed remaining in the customs union.

It is likely to table an amendment to the Government's Brexit legislation, in which Tory rebels could join the opposition to inflict a defeat.