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."

Mr Tusk is undertaking a whirlwind tour of EU capitals including Berlin, Paris and Athens to sell the package of reforms he drafted in response to Mr Cameron's demand for change.

Britain's renegotiation is the first item on the agenda for the two-day European Council summit in Brussels, meaning agreement could be reached as early as Thursday evening.

However, the gathering of 28 EU leaders is not scheduled to conclude until Friday lunchtime, after which Mr Cameron will call an immediate Cabinet meeting if he secures a deal.

The meeting will effectively fire the starting gun on the referendum battle, as Eurosceptic ministers will be given the green light to campaign for a Leave vote in the poll expected on June 23.

Mr Cameron had come under pressure from ministers to bring Cabinet forward as Eurosceptics feared the Remain campaign would have an advantage if collective responsibility was not suspended until the next scheduled meeting of the PM's top team on Tuesday February 23.

They believed the Remain campaign would steal a march in the vital first few days of the campaign by leaving the PM free to proclaim the merits of the deal in weekend TV interviews and a statement to the Commons on Monday, while colleagues remained gagged.

Agreement this week remains far from certain, with Downing Street confirming that key details still need to be "pinned down". Foreign Secretary Philip Hammond, who was meeting EU counterparts in Brussels, said there were still "very big issues" on the table.

If concerns from eastern European states over restrictions to migrant worker benefits and French reservations about financial regulation cannot be allayed, Mr Cameron has made it clear he is ready to wait until the next summit in March to try again for a deal, rather than accept one that falls short of his demands.

If the summit runs over into Friday evening or Saturday, the PM will review whether to call ministers to Number 10 for a weekend Cabinet meeting.

Mr Cameron is heading to Paris for talks with Mr Hollande on Monday evening about French concerns that proposed protections for non-eurozone states could amount to special treatment for the City of London.

The PM's official spokeswoman declined to say whether the hastily arranged meeting was requested by the UK over fears that France may block a deal in Brussels.

The spokeswoman said: "What we have seen from the French throughout, and from President Hollande personally, is the French wanting to work with us so we can both agree on the solutions that the UK needs."

Meanwhile, Mr Cameron was branded a chicken by Nigel Farage after Number 10 revealed the PM will not meet the Ukip leader during a visit to Brussels on Tuesday.

The PM had been expected to meet leaders of all the groupings in the European Parliament, which will eventually have to approve parts of the reform package including restrictions on EU citizens' benefits.

Instead he will meet European Parliament president Martin Schulz, leaders of the centre-right EPP and the Socialist grouping, and three MEPs who have been acting as "sherpas" in the negotiations, rather than all group leaders.

Asked whether MEPs could be in a position to block the changes after the referendum, the PM's spokeswoman said: "This would be a legally binding document under international law, entered into by the 28 leaders of member states. There are a number of decisions subsequently which would have to be delivered by secondary EU law by co-decision with the European Parliament."

She added: "We will be looking for all countries in the European Parliament to deliver on that, following the decision of the British people."