The Northern Ireland Protocol has faced criticism since its implementation at the beginning of the year. Boris Johnson and Lord Frost have held numerous meetings with EU counterparts to resolve the huge problems that have emerged – but to no avail.

Briefing MPs on Parliament’s European Scrutiny Committee on Monday, Lord Frost said talks had not been “hugely productive”.

Now US senators on Capitol Hill have waded in and insisted the Protocol protects the Good Friday Agreement, agreed in 1998, to ensure peace and prosperity in Northern Ireland after the Troubles.

The conditions for a US-UK trade deal were put forward by Democratic Senator Bob Menendez, chairman of the influential Senate Foreign Relations Committee.

The Northern Ireland Protocol was incorporated into the Brexit Withdrawal Agreement to help avoid a hard border on the island of Ireland.

But it means Northern Ireland remains in the EU single market resulting in regulatory checks being undertaken on select goods coming in from Britain.

Loyalists and Unionist politicians at Stormont have also expressed anger at what they regard as a border in the Irish Sea and separation from the rest of the UK.

Putting forward the resolution which was agreed unanimously, Mr Menendez, said: “The successful negotiation of the Good Friday Agreement stands as a historic and groundbreaking success that has proven critical to the decades of relative peace that have followed.

“The reintroduction of barriers, checkpoints, or personnel on the island of Ireland, also known as a “hard border”would threaten the successes of the Good Friday Agreement.

“The United States Congress played a prominent role in support of negotiations of the Good Friday Agreement and has taken a leading role in promoting peace on the island of Ireland more broadly.”

The Democratic Senator, who is seen as an influential figure on directing foreign policy in the Joe Biden administration, acknowledged the US had a “special relationship” with the UK on “trade issues”.

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But he stressed: “The Senate urges the United Kingdom and the European Union to support peace on the island of Ireland and the principles, objectives, and commitments of the Good Friday Agreement.

“[We] will take into account, as relevant, conditions requiring that obligations under the Good Friday Agreement be met as the United States seeks to negotiate a mutually advantageous and comprehensive trade agreement between the United States and the United Kingdom.”

Express.co.uk understands the move has put new pressure on Trade Secretary Liz Truss and Minister of State Lord Frost during negotiations with United States Trade Representative Katharine Tai and the European Union.

Rebecca Ellis, Lord Frost’s senior official on Northern Ireland said discussions with the European Commission were “actively ongoing”.

Meanwhile, Ms Tai this afternoon held talks with Liz Truss.

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During the call, both sides discussed ways forward to resolving a dispute over aircraft subsidies which would help to put securing trade agreement “on the right track”.

Speaking at the House of Commons International Trade Select Committee ahead of her crunch conversations, Ms Truss described the United States as a “close ally” and appeared to show signs of a positive attitude to her US counterpart.

Sources close to the International Trade Secretary told Express.co.uk conversations were “positive” and added: “It will be one step at a time as we overcome the hurdles one by one.”

When asked about issues surrounding the Protocol and the prospect of a deal, a senior source said: “We are closely working with Lord Frost and the Cabinet Office on this.”

The White House today said they supported the Good Friday Agreement with senior officials telling this publication that it was “essential the agreement isn’t compromised.”

Asked about the prospect of a US-UK trade deal being jeopardised because of the protocol, a senior White House official added: “A secure and prosperous Northern Ireland is what we want.

“The provisions in the EU-UK trade and cooperation agreement and the Northern Ireland protocol ensure this.

“We are watching closely the ongoing negotiations the UK is having with the EU on this matter.”

In a statement, Ms Tai added: “Improving our trade relations with trusted Allies and partners will not only improve our prosperity but our national security. 

“We are also working with the European Union and the United Kingdom to resolve the ongoing Boeing-Airbus dispute.

“These talks will take time, but I believe a resolution is possible and worth pursuing.”