David Cameron is making final efforts to bolster support for his proposed European Union reforms both in Brussels and within his own party before a crunch summit.
As negotiations continued over the details of the package, the Prime Minister was boosted by Angela Merkel's declaration that most of his demands were "justified and necessary".
The German chancellor sounded an upbeat note about the chances of the leaders of the 28-member bloc agreeing a package when they gather for a two-day meeting on Thursday.
With the latest draft due to be put out later by European Council president Donald Tusk, Mr Cameron still faces some tough wrangling, not least with Eastern European counterparts over curbs to benefits for migrant workers.
Agreement at the summit would allow an in/out referendum to be held as early as June and open the door for senior Eurosceptic Tories to join the campaign in favour of Brexit.
Among prominent figures yet to show their hand is London Mayor Boris Johnson, who was called to Number 10 for talks as the PM seeks to secure his support for the "remain" camp.
Mr Johnson remained tight-lipped on his stance as he emerged from the 40-minute discussion but senior sources said he would "make everything abundantly clear by the end of the week" if a deal was concluded by EU leaders.
They insisted he remained "genuinely conflicted" on the issue and rejected suggestions he was seeking to exploit the issue to maximise his chances of succeeding Mr Cameron as Conservative leader.
If he secures a deal Mr Cameron will return to the UK for a Cabinet meeting to agree a Government position - and he has told ministers that collective responsibility will then be suspended on the issue so that they can campaign for either side.
Downing Street conceded there "are still details to be nailed down" to secure an agreement this week but insisted Mr Cameron's talks with key figures in Brussels on Tuesday had been "useful".
In a pre-summit speech to the German parliament, Mrs Merkel said the changes being sought by Mr Cameron were "far from being demands that are just for Britain" and that it was in her country's national interest for the UK to remain in the EU.
"They are also European demands and many of them are justified and necessary," she said.
"Germany will make its contribution so that a result that satisfies everyone can be achieved, if possible already at the summit beginning tomorrow," she went on.
"I am convinced that it is in our national interest for Great Britain to remain an active member in a strong and successful European Union."
Alongside last-minute wrangling over the EU deal, Mr Cameron is also finalising new domestic legislation designed to assert the sovereignty of the UK Parliament.
The Prime Minister is expected to publish plans soon for the UK Supreme Court to act as a "constitutional longstop" to regulate the impact of EU law on the UK in a way already done in Germany.
It is one of several areas of concern set out by Mr Johnson, who has questioned whether it would be a "bazooka or popgun" in taking on European judges and bureaucrats.
Tory former attorney general Dominic Grieve has suggested any such move would be "pointless" as it could be overridden by the UK's treaty obligations.
Conservative MP John Baron complained of "poor" communication between Downing Street and party backbenchers over the issue - after a letter signed by 40 colleagues failed to secure talks with the PM.
"We've been unsuccessful in obtaining that meeting so I haven't been privy to any discussions. Boris Johnson apparently has, I wish him well," he told BBC Radio 4's The World At One.
"But unless we grasp this fundamental concept of do we have the ability to stop any unwanted EU laws, regulations or taxes, then we're fudging the issue. Anything else is a smokescreen and that stark choice will be evident for all to see when it comes to the referendum."