THE UK Government would face legal action for breaching international law if it left the EU without a deal, according to a senior politician in Brussels.

Philippe Lamberts, a member of the European Parliament’s Brexit Steering Group, warned that crashing out of the bloc would be breaking the terms of the Good Friday Agreement.

He said as well as undermining the peace process in Northern Ireland the move would likely lead to a lawsuit against the UK Government brought by the Republic of Ireland.

“The possibility is still very much there of a no-deal Brexit,” he said.

“It does breach the Good Friday Agreement and one state carries responsibility for that and it is the UK. The decision to launch Article 50 was a unilateral decision by the United Kingdom and the UK should keep in mind ... the EU is not party to the Good Friday Agreement but the UK is.”

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Asked what could be the legal repercussions of breaching the Good Friday Agreement, he said: “I would say so, that other parties – particularly the Republic of Ireland – would be in a position to take action against the United Kingdom.”

He added that the consequence of this would be both a lengthy legal battle between the Republic of Ireland and the UK, and also a diminishing of trust in the UK on the international stage.

“I would doubt this [legal case] would generate any quick results, but [also] consider the effect of the quality of signature of the UK under any international treaty moving forward,” he said.

“The Brexiteers dreamed that they are going to strike free trade agreements with the United States and whoever else, that they were going to replace the 600-plus treaties which the EU has around the world ... if the UK shows how little it cares for an international treaty to which it is signed up – the Good Friday Agreement – who can still believe in the validity of the United Kingdom?”

Lamberts went on to say that if the UK opposed the backstop it was up to it to suggest an alternative which maintained an open border in Ireland.

“You cannot expect people outside of the UK to solve the problems created by the UK. If the UK is removing one of the foundations of the Good Friday Agreement – membership of the EU – it has to replace it with something that performs the same function,” he said.

The Good Friday Agreement officially known as the Belfast Agreement provided the framework for the peace process in Northern Ireland, helping to bring to an end the Troubles which began in the late 1960s and during which more than 3500 people were killed.

It was signed by former Labour prime minister Tony Blair and former Irish premier Bertie Ahern in Belfast in 1998, pictured left.

The agreement provided a way of managing divisions between nationalists and Unionists in Northern Ireland by allowing people to lead their lives and to do business as they chose on a north-south and/or east-west basis. Such an open situation was possible because both the UK and Irish Republic are in the EU.

May is already facing a legal challenge from Lord Trimble, a former leader of the Ulster Unionist Party, over how Brexit impacts on the agreement. He is working on the plans to challenge the withdrawal agreement in the High Court in London.

One academic source supported a possible challenge to the UK Government from the Irish government if there was no deal.

He said the case would likely be taken to the International Court of Justice in the Hague which settles legal disputes between member states.