The European Union has stepped up planning for a "no-deal" Brexit after British Prime Minister Boris Johnson's government refused to revoke a plan to break the divorce treaty that Brussels says will sink four years of talks.

Britain said explicitly this week that it plans to break international law by breaching parts of the Withdrawal Agreement treaty that it signed in January, when it formally left the bloc.

Britain says the move is aimed at clarifying ambiguities, but it caused a new crisis in talks less than four months before a post-Brexit transition period ends in December.

European Parliament politicians have said they will not approve any new EU-UK trade deal unless Britain fully implements the Withdrawal Agreement.

The chamber must approve any such trade deal for it to be enacted.

If the UK authorities breach the divorce deal, or threaten to do so, then "the European Parliament will, under no circumstances, ratify any agreement between the EU and the UK," the parliament's Brexit group and the heads of the parliamentary political groups said in a statement.

Earlier Minister for Foreign Affairs Simon Coveney said the best case scenario the Irish Government could hope for with Brexit is a "very basic and pretty thin agreement", which he said would cover fisheries, avoid tariffs and quotas and deal with issues of dispute.

Speaking at the Agriculture Science Association's annual conference, Mr Coveney said a no deal situation would mean tariffs on Irish exports to the UK of €1.3bn to €1.5bn, which he said would be a "massive problem" in particular for beef and dairy exports.

Britain must respect its commitments in the withdrawal treaty it signed with the European Union if it wants to have an agreement on future trade relations with the bloc, the chairman of eurozone finance ministers Pascal Donohoe has said.

In one of the most extraordinary turns since the 2016 Brexit referendum, Britain explicitly said this week that it plans to break international law by breaching parts of the Withdrawal Agreement treaty it signed in January. 

This plunged talks on a future trade relationship between the EU and Britain into crisis less than four months before the UK is due to leave the European Union at the end of a transition period. 

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"As the UK looks to what kind of future trade relationship it wants with the European Union, a prerequisite for that is honouring agreements that are already in place," Finance Minister Pascal Donohoe told reporters before the euro zone ministers' talks.

The European Commission, which conducts talks on the future trade deal with London on behalf of the 27-nation bloc, asked Britain yesterday to drop by the end of September the parts of a planned bill that would break the treaty with the EU. 

"It is imperative that the government of the United Kingdom respond back to the call from the Commission ... this is a prerequisite to what any future relationship could look like," Mr Donohoe said. 

German Finance Minister Olaf Scholz, also speaking to reporters, said the EU would be open to a deal with Britain until the "last moment", but Britain had to respect what it had agreed to. 

"Treaties have to be respected - anyone knows that. My view on the trade discussions is the following: You have to do it up to the last moment, but you have to be very clear," Mr Scholz said. 

The bill Britain wants to pass would override provisions in the withdrawal treaty meant to ensure Britain would not give companies exporting to the EU an unfair advantage by subsidising them. The EU is sensitive to keeping competition with Britain fair after Brexit.

"We will not accept anything that could jeopardise or weaken the single European market," French Finance Minister Bruno Le Maire said.

Earlier, Taoiseach Micheál Martin told the ASA's conference in Cork that the current uncertainty around Brexit is very "unhelpful" to the agri-food sector in Ireland.

He said he has told British Prime Minister Boris Johnson that a free trade outcome was optimal for Ireland.

He said the current "undermining of the Northern Ireland" protocol was a source of concern.

It comes as Minister of State for European Affairs Thomas Byrne said the UK's claim that their plans to override key elements of the Brexit Withdrawal Agreement is to help the Good Friday Agreement is "completely false".

Yesterday, British Cabinet Office Minister Michael Gove - who co-chairs a joint committee on the Withdrawal Agreement - said the government was not prepared to back down on the plans.

The Internal Market Bill will see the UK government reserve the right to unilaterally interpret the Northern Ireland protocol's rules on state aid and customs declarations.

Speaking on BBC Radio 4, Mr Byrne said one can expect "moments of anguish and drama" during Brexit negotiations, but the latest move taken by the British government is unacceptable.

He said Ireland values good relations with Britain but this was a "unilateral provocative act" taken by the British government in changing their approach to the Withdrawal Agreement.

He said the treaty had already been put in place so Ireland is very concerned.

"We have never in dealing with other countries have seen such a 'renegement' on an agreement less than one year after Boris Johnson negotiated it. It is unacceptable," he said.

"This protocol exists and one side cannot make it not exist."

"What they propose to do is put at serious risk the basis of the peace on the island of Ireland.

"And the basis of our trade, and unfettered trade, cross border in goods which is absolutely essential for that peace." 

Mr Byrne said it is completely wrong to say this changed approach is to protect the Good Friday Agreement as it does the opposite.

He also questioned how other countries will rely on commitments that the UK makes.

He said what should happen now is the UK should withdraw the legal provisions that they plan to enact.

He said the UK government is putting at serious risk the peace on the island of Ireland and unfettered cross border trade on goods which is "essential for that peace and for day to day life".

He said talks are continuing and Ireland stands fully behind the EU negotiating team and support them.