David Cameron's renegotiation of Britain's EU membership is "very fragile" and unless handled carefully could lead to the break-up of the union, European Council president Donald Tusk has said.
Mr Tusk said it was a "critical moment" as he told EU leaders to start listening to each other's concerns ahead of this week's crunch Brussels summit where the Prime Minister is hoping to secure a final deal.
He warned leaders that "what is broken cannot be mended" and the "risk of break-up is very real".
Following a meeting with Romanian President Klaus Iohannis, Mr Tusk said: "After my talks today with President Iohannis, Prime Minister Cameron, and (French) President (Francois) Hollande, and before meeting Greek prime minister (Alexis) Tsipras, (Czech) prime minister (Bohuslav) Sobotka and (German) chancellor (Angela) Merkel tomorrow, I have only one political reflection: this is a critical moment.
"It is high time we started listening to each other's arguments more than to our own.
"It is natural in negotiations that positions harden as we get closer to crunch time. But the risk of break-up is real because this process is indeed very fragile. Handle with care. What is broken cannot be mended."
He added: "These are difficult but also hugely important negotiations.
"At stake is the United Kingdom's membership of the EU. A question which only the British people can and will decide.
"At stake is also the future of our European Union where we will all have to decide together, and where we cannot and will not compromise on the fundamental freedoms and values. It is in this spirit that I drafted my proposal for a new settlement for the UK in the EU.
"There are still a number of outstanding political issues that we will have to agree on. These include the questions of future treaty change, a so-called emergency brake for non-euro area countries, a safeguard mechanism on access to in-work benefits, and finally the notion of ever closer union."