The US Congress will not support any post Brexit trade deal with the UK that undermines the open border on the island of Ireland, Taoiseach Leo Varadkar has said.

Mr Varadkar said a UK-US trade deal - the goal of many of the strongest supporters of Brexit - will not be agreed if there is a return to a hard border.

The Taoiseach’s comments come amid a continuing impasse in the Brexit process over the backstop, the insurance policy to avoid a hard border if there is no future EU-UK trade deal.

The backstop is the main stumbling block to the Brexit withdrawal agreement passing in the House of Commons.

Speaking at a Fine Gael event on Friday evening, Mr Varadkar said: “Westminster might be the mother of all parliaments but it’s not the boss of other parliaments and I know the European Parliament will not vote for a withdrawal agreement that does not do right by Ireland.

“And I don’t believe the United States Congress will agree to any trade between the US and the UK that would undermine the Good Friday Agreement and the open border between North and South.”

Tánaiste Simon Coveney recently visited Capitol Hill to rally support for Ireland’s position on Brexit.

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Speaking to The Irish Times, Congressman Brendan Boyle said: “Peace in Ireland is a paramount concern to me. Therefore, I would not sign up for any trade deal that would undermine the Good Friday Agreement.”

Mr Boyle is a member of the Ways and Means Committee - a congressional committee that has oversight over any trade deals signed between the United States and third countries.

The committee is chaired by Richard Neal, a congressman with a keen interest in Irish-American affairs having been centrally involved in US efforts during the peace process in the mid-1990s.

Selection convention

Last week Britain and the US signed a “Mutual Recognition Agreement” which replicates the existing trade agreement between the US and the EU on technical standards applied to traded goods from each country.

But negotiations on a long-term trade deal between the two countries will only begin after Britain leaves the EU and any future trade deal will need the approval of Congress.

Discussions on the state-of-play with Brexit are likely to dominate Mr Varadkar’s trip to Washington next month, which will take place less than two weeks before Britain is due to leave the European Union at the end of March.

The Taoiseach will meet other European leaders at an EU-Middle East summit in Egypt this weekend.

Mr Coveney said the Government would seek to avoid a hard border if there is a no-deal Brexit by insisting the UK government honour a December 2017 commitment for North-South “regulatory alignment if no other mechanism works.”

This solution, essentially a type of backstop applying specifically to Northern Ireland rather than to the whole of the UK - the option in the withdrawal agreement - was rejected by pro-Brexit Tory MPs and the DUP at the time.

DUP MP Ian Paisley said on Friday the current backstop or any reversion to the proposed December 2017 proposal would be unacceptable.

The Taoiseach, speaking at a Fine Gael European Parliament selection convention, also suggested Fianna Fáil may not have extended the confidence and supply deal last year if it was ahead of Fine Gael in the polls.

“I recognise the role they have played in providing political stability given the uncertainty of Brexit,” he said. “But is there a sting in the tail? I wonder, if FF was 5-10 per cent ahead in the polls for the last year, would they be quite so willing to facilitate us?”

His comments are likely to be criticised by Fianna Fáil, which is holding its ard fheis in Dublin this weekend. Fianna Fáil leader Micheál Martin on Friday criticised the Government’s preparations for a no-deal Brexit.