EUROPEAN leaders have warned Boris Johnson his "farcical" plan to renege on the Brexit withdrawal agreement could end up backfiring by strengthening EU resolve and solidarity.

Diplomats, politicians and officials in Brussels expressed shock at the Prime Minister's plans to override aspects of the treaty in which the Prime Minister agreed to checks on goods between Northern Ireland and Britain, in order to preserve an open border on the island and on the ongoing peace process.

Bernd Lange, the German MEP and chair of the trade committee in the European Parliament, said he was “shocked” by the UK Government’s approach to the withdrawal agreement and that he has “never seen anything like it in decades”.

"There were signs that the joint political declaration for Boris Johnson was not worth the paper it was on. Now he’s said it. Boris Johnson is thus turning the negotiations up to now and the serious efforts of the European Union into a farce," he said.

"We will not allow ourselves to be divided by these tactical games, but will stick to our previous constructive but determined approach - in the spirit of the joint political declaration of eight months ago, which both sides had signed. We stick to agreements. The European Parliament reaffirmed its negotiating position in a resolution in July. There cannot and will not be an agreement at any price.

"What madness to believe today that one can achieve actual sovereignty in complete independence. In today’s sea of globalisation with powerful global players, sovereignty can only be developed together. This shows the success story of the EU and many countries that have concluded treaties and keep them. The British illusion of sovereignty will lead to the greatest loss of sovereignty in British history."

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Ursula von der Leyen, the European Commission president, warned a No-Deal Brexit would be the consequence of any backtracking, saying it is a “prerequisite” in order for the UK to get a free trade deal.

She tweeted: "Trust the British government to implement the Withdrawal Agreement, an obligation under international law & prerequisite for any future partnership. Protocol on Ireland/Northern Ireland is essential to protect peace and stability on the island & integrity of the single market."

A spokesman for the European Commission added: "The full implementation of the withdrawal agreement and in particular the protocol on Ireland and Northern Ireland are essential.

"These are legal obligations under international law, this is a matter of trust, this is a prerequisite, a precondition for the negotiations on the future partnership. I think that’s clear and I don’t think we need to say any more.”

The internal market bill, to be published on Wednesday, would “clearly and consciously” undermine the agreement on Northern Ireland by revoking requirements for checks between the islands and an obligation for EU state aid rules to apply to the region, the Financial Times wrote, citing people familiar with the plans.

The report, which was not denied by Downing Street, hiked tensions ahead of the start of the latest round of Brexit talks in London on today.

The UK Government yesterday released comments from a speech to be given by Johnson, in which he declares that not reaching a deal “would be a good outcome for the UK”.

Johnson's chief negotiator David Frost said on Sunday the UK was not “scared” of a crash-out exit and Britain was going to be “an independent country” and not a “client state”.

European leaders hit back saying they were fully independent as EU members. Germany’s ambassador to the UK Andreas Michaelis said Germany had signed “hundreds of international treaties” and was still a sovereign nation.

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Diplomats in Brussels expressed disbelief at the developments.

"The Frost-Johnson sabre rattling was wholly expected hullaballoo before this round, but the changing of the foundation of the withdrawal agreement would be an incomprehensible miscalculation,” an EU diplomat told The Irish Times. "Unless of course the UK is not looking to take back control but just isolate itself entirely on this continent.”