Video: 'Crunch Time' For Cameron Over EU

Mr Cameron has also been the first British Prime Minister to visit Austria in 20 years and the first to visit an independent Slovenia.

"He has got really stuck in," an official said.

Before the summit gets under way, Mr Cameron will have another meeting with Mr Tusk, who as President of the Council will chair the talks and - worryingly for the Prime Minister, perhaps - has already warned that there is "no guarantee that we will reach an agreement".

But, in an eve-of-summit letter, Mr Tusk said: "There will not be a better time for a compromise."

He said failure to reach a deal would be "a defeat both for the UK and the European Union, but a geopolitical victory for those who seek to divide us", in what was seen as a reference to terror groups like Islamic State and possibly President Putin of Russia as well.

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British Government officials claim "a lot of progress" has been made on issues that some had said would never be possible, such as removing "ever closer union" from Britain's terms of membership and benefit restrictions.

But officials admitted the main issues of disagreement were treaty change, the so-called "emergency brake" on welfare benefits - "who triggers it, how it is triggered, how long it remains triggered" - and curbs on child benefits to workers from Eastern Europe.

Worryingly, perhaps, a draft of the deal that was due to be published on Wednesday has apparently been delayed.

Britain's renegotiation will not be the only major item on the agenda in Brussels, however. Europe's migration crisis will be uppermost in many leaders' minds, with 2,000 migrants a day arriving in Turkey and Greece in the winter months and fears that the push on Aleppo in Syria could make mater worse.

Lengthy discussions in Brussels on migration could delay a deal on Britain's new membership deal.

Back in Downing Street there is already some talk that Mr Cameron's proposed post-summit Cabinet meeting, at which he plans to seek backing for his deal but also suspend collective responsibility so that ministers are finally free to campaign to leave the EU, may have to be held on Saturday instead of Friday afternoon.