The Irish border backstop would constrain the UK’s negotiating position during post-separation trade talks, a pro-Brexit legal expert said.

Martin Howe QC, who chairs Lawyers for Britain, said there would be no incentive for the EU to make concessions if the backstop was the default.

Another barrister who gave evidence to a parliamentary inquiry into the legalities behind the measure said the bloc wanted to strike a deal.

Mr Howe said: “You cannot hold out for your essential interests.

“If you fail to reach agreement these terms come into effect, that constrains your negotiating power.

“Why should the other side give you better terms unless you are able to offer them something else in return that is better for them?”

He gave evidence to the Northern Ireland Affairs Committee of MPs at Westminster.

Labour MP Kate Hoey summarised his comments: “We are going in with our hands tied behind our back.”

The backstop, contained in the proposed EU withdrawal treaty, has been characterised as an insurance policy which neither side wants to use.

It would come into effect only if no better deal is struck during trade talks after Brexit.

Mr Howe said: “You cannot really argue that even if it does not come into force it does not have a very, very severe constraining effect on the UK’s negotiating position.”

He added that the draft protocol created more uncertainty.

“The danger is that the backstop gets triggered if they are not finished by the end of 2020.

“It makes it impossible for the UK to conclude and negotiate trade treaties with third parties because we don’t know when you can implement them.”

Legal counsel Isabelle Van Damme said the UK and EU’s economies had historically been well integrated.

Isabelle Van Damme (Parliament/PA)

She challenged the assumption that Europe was not interested in striking a deal.

“I have a bit of a difficulty with accepting that as a starting point because there are so many matters in which the EU and the 27 member states co-operate to the advantage of the respective economies.”

Mr Howe, nephew of former Conservative chancellor in the Thatcher administration, Geoffrey Howe, addressed the impact of a no-deal Brexit on the Republic of Ireland.

Ireland uses Great Britain as a relatively cheap landbridge to transit goods from mainland Europe.

He said: “In a no-deal scenario, whatever else happens, it will be important to come to arrangements with the Republic of Ireland which smooth the flow of such transit traffic.”

He added that the EU should be encouraged not to unnecessarily obstruct British lorries going through Calais.