The Taoiseach has said he hopes the UK's Brexit proposals contain new thinking as he issued his strongest warning yet that Europe will not compromise on its fundamental principles.
Leo Varadkar received a briefing from Prime Minister Theresa May on the fringes of a Brussels meeting of European leaders about the UK's forthcoming white paper aimed at breaking the deadlock in the negotiations.
He remained tight-lipped about its draft contents but publicly warned his British counterpart against compromising or cherry-picking elements of the EU's four freedoms of goods, services, capital and people during forthcoming talks over the summer.
Mr Varadkar said: "That would be the beginning of the end of the Single Market.
"While we regret them leaving we are not going to let them destroy it."
Mrs May's Cabinet is split over whether to form a Customs Partnership with the 27-member bloc after Brexit or introduce a customs border but use technology and special arrangements like trusted trader status to minimise its impact.
Ministers are due to meet soon in a bid to reconcile their differences.
Mr Varadkar said: "I am an optimist, I imagine that the white paper will contain new thinking."
He added: "I do sincerely hope that it can be the basis for negotiations to begin on the final stages of the (Withdrawal) Treaty.
"There is a landing zone on which that has to land and cannot involve cherry-picking the four freedoms."
He refused to countenance the UK having access to the market for goods but not capital or free movement of labour.
If that were to happen Eurosceptics elsewhere in Europe would seek the same, the Taoiseach envisaged.
EU-27 leaders attended a Brussels meeting of the European Council on Brexit on Friday after the departure of the British Prime Minister.
The future of the invisible and frictionless 300-mile frontier between Northern Ireland and the Republic is one of the most vexed issues still facing the UK and EU negotiators.
If no deal is struck by this autumn a backstop would be introduced.
That has been interpreted by Europe as meaning Northern Ireland would continue to follow the rules of the EU Customs Union during a transitional phase.
There has been no agreement on the detail of the provision, with Europe calling for more substantial progress but the Democratic Unionists adamantly opposed to anything which would create a regulatory difference between Northern Ireland and the rest of the UK.
The UK Government is against creating any infrastructure on the border and has promised to bring forward further proposals on trade soon.