Leo Varadkar has called on Boris Johnson to dial down the “nationalistic rhetoric” and avoid putting down “rigid red lines” early on in the Brexit trade talks.
The Irish premier made the plea after Mr Johnson accused Brussels of attempting to "change the terms" of the Withdrawal Agreement and Political Declaration since they were signed off last year.
"I've learned a lot from the past two and a half years dealing with Brexit and dealing with two different British prime ministers," the Taoiseach said.
"And one thing I'd say to everyone is let's not repeat some of the errors that were made in the past two-and-a-half years.
"Let's not set such rigid red lines that it makes it hard to come to an agreement and let's tone down the kind of nationalistic rhetoric."
“As is always the case when it comes to negotiations, setting out so boldly such firm red lines actually makes coming to an agreement more difficult because the other party you are negotiating with doesn’t feel they got a fair deal unless those red lines get turned pink or bent in some way.”
Varadkar also took aim at a leaked order for British diplomats to sit separately from their EU counterparts at international summits – calling it “petty” and likening the move to "primary school" behaviour.
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He said: "I think it just comes as being a little bit petty.
"It's kind of when you're in primary school or in secondary school that you get worried about who you sit beside in class.”
Mr Johnson is expected to flesh out his plans for a post-Brexit relationship with Brussels in a speech on Monday ahead of the start of trade talks in March.
According to PoliticsHome, Mr Johnson will reportedly call for the UK to be treated as an "equal" in the talks and demand "no alignment, no jurisdiction of the European courts, and no concessions".
Foreign secretary Dominic Raab said on Sunday that any suggestion the UK will have to follow European Union rules and laws after 2021 “just ain’t happening”.
Raab used broadcast interviews on Sunday morning to insist that Britain would no longer be a rule-taker now Brexit had been delivered.
“We’re not going to be aligning with EU rules, that’s not on the negotiating table, it’s not even an issue of red lines – it is not even in the negotiating room,” he told the BBC’s Andrew Marr.
“We are entering into these negotiations with a spirit of goodwill.
“But we are just not doing that other stuff. The legislative alignment, it just ain’t happening.”