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The opening salvo takes place on the eve of critical post-Brexit trade talks that will determine Britain's future trade relationship with the European Union from early next year. The next round of talks will begin on Monday and will be held face to face for the first time since the coronavirus epidemic. David Frost, known as "Sherpa" in EU negotiations, said that the "intensified process" in the discussions had to be realistic.
The comments come after the Prime Minister insisted that Britain would reject an EU offer to extend the Brexit transition period beyond the end of the year.
In Bullish, Mr. Frost insisted that the UK's sovereignty over laws, courts and fish waters "is not up for discussion".
He also said that what he called some of the EU's "more unrealistic positions" had to change to make progress.
In a tweet, Mr. Frost said: “We will go to Brussels in good faith to deal with the EU's concerns.
“This has to be a real negotiation and some of the EU's unrealistic positions have to change if we want to move forward.
“We have carefully read what the EU has said on the subject over the past few days and look forward to discussing it.
“The UK's sovereignty over our laws, our courts or our fishing waters is of course not up for discussion.
Michel Barnier (right) and British chief Brexit negotiator David Frost (Image: Dati Bendo / DPA / PA images)
“Likewise, we are not looking for anything that would undermine the integrity of the EU internal market.
"I want to make it clear that the government will not agree to ideas like the one currently in circulation that will give the EU a new right to retaliate against tariffs if we enact laws that are in our interests.
"We couldn't face such an unpredictable economic risk."
Mr. Frost said he was looking forward to resuming personal discussions after the coronavirus outbreak.
He said: “The next round of talks with the EU starts on Monday, June 29th. This is the beginning of the intensified process.
“For the first time since March, we will meet face to face in Brussels. We look forward to welcoming the EU team to London the following week.
"These meetings will be smaller and focus on whether we can start making real and rapid progress toward an agreement."
Britain has a deadline of July 1st if it wants to extend the transition period beyond the end of the year.
Mr Frost's EU counterpart said Britain must send "clear signals" that it wants to seal an agreement with the European Union on its post-Brexit relationship.
Michel Barnier said the UK had not previously dealt with preliminary openings by the EU on state aid and fisheries in previous rounds of negotiations.
In an online seminar, Barnier added: "The ball is in the British court, I think the deal is still possible."
He said he was "disappointed" with Britain's refusal to negotiate foreign policy and defense, but he was open to "room for flexibility" in the EU's and UK's previously conflicting positions on fisheries and fair play guarantees to find for state aid.
When asked how far the bloc could go to the UK in terms of the so-called level playing field for fair competition, Barnier added: “In addition to fishing and governance, we are ready to work on landing zones, and the mandate of respect EU. "
Tuesday was the fourth anniversary of the historic referendum, in which 17.4 million people voted to leave the EU.
Conservative MP Michael Fabricant said: "We need to hold on to our arms and not extend the transition so that the EU can focus on a new, mutually beneficial trade deal with Britain while maintaining our sovereignty as a major power outside the EU."