The battle over Britain's future in the EU intensified Wednesday as Prime Minister David Cameron braced for a barrage of attacks from eurosceptics over proposed reforms the European Commission defended as a "fair" deal for all.
Speaking at the European Parliament in Strasbourg, Juncker said the proposals due to be put to a key summit this month were "fair" for Britain and its 27 partners.
EU president Donald Tusk unveiled proposals to avoid Britain leaving the club on Tuesday, firing the starting gun on two weeks of tense negotiations to reach a deal at the summit of EU leaders.
The plans include a four-year "emergency brake" on welfare payments for EU migrant workers, protection for countries that do not use the euro and a "red card" system giving national parliaments more power.
Cameron on Tuesday said Tusk's plans showed "real progress" and made it likely that he would campaign to stay in the European Union in a referendum expected in June.
But he will likely face tough questions from British lawmakers in a debate due to start at 1230 GMT.
Many have greeted the plans with scorn, with UK Independence Party head Nigel Farage dismissing them as "pathetic," while Conservative lawmaker Steve Baker said the prime minister was just "polishing poo".
Former defence secretary Liam Fox warned that up to five members of Cameron's cabinet could campaign to leave the union having seen the proposals.
London's flamboyant mayor Boris Johnson has been named in the press as a potential rallying point for eurosceptic ranks.
However, the prime minister received a boost by winning the backing of interior minister Theresa May, a known eurosceptic.
Britain's newspapers on Wednesday poured cold water on the plans, with popular tabloid The Sun splashing "Who do EU think you are kidding Mr Cameron?" across its front page, saying the deal was a "farce".
The Daily Mail called it "The Great Delusion" and the eurosceptic Daily Express "A Joke" on their front pages.
The proposals are also expected to be a hard sell for some EU states, which fear Cameron is winning too many concessions ahead of a February 18-19 summit.
Negotiations are set to begin at the European Parliament on Wednesday, while Cameron is set to present the plans to parliament after the weekly Prime Minister's Questions session.
He then begins a charm offensive that will take him to Poland and Denmark on Friday then Germany next week.
London's bid to transform its EU membership has added to turmoil in the bloc as it struggles with its worst migration crisis since World War II and the fallout from the eurozone debt saga.
The British premier said Tusk's proposal showed he had "secured some very important changes".
"If I could get these terms for British membership I sure would opt in for being a member of the EU," Cameron said in a speech in southwest England.
But British eurosceptics were unconvinced, with Johnson saying he had "doubts" about their effectiveness.
Opinion polls are split on whether Britons would back leaving the EU in their first vote on the subject since 1975.
Tusk's most controversial proposal is an "emergency brake" that would allow any EU state to limit the welfare payments migrants from other European countries can claim for up to four years after their arrival.
To pull the brake, states would have to prove an "exceptional situation" in which their welfare system and public services are overwhelmed, get approval from the European Commission and then from other bloc leaders in a majority vote.
Despite concerns in France, Tusk's plan also includes a "mechanism" by which the nine countries that are not in the euro can raise concerns about decisions by the eurozone.
But he stressed that the mechanism could not delay or veto urgent decisions by the 19 countries in the euro.
The "red card" system would allow a group representing 55 percent of the EU's national parliaments to stop or change draft EU laws.
Although Cameron has only set a deadline of the end of 2017 to hold a referendum on whether Britain should stay in the EU, sources have said he is keen to push a vote through by June.
That would avoid the fallout from any new flare-up in Europe's migration crisis this summer and British eurosceptics becoming even more unruly.