EU president Donald Tusk unveiled proposals to avoid Britain leaving the 28-nation club yesterday, firing the starting gun on two weeks of tense negotiations to reach a deal at a summit later this month.
They 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 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.
They 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 today, before Cameron begins a charm offensive that will take him to Poland and Denmark on Friday then Germany next week.
"To be, or not to be together, that is the question which must be answered not only by the British people in a referendum, but also by the other 27 members of the EU in the next two weeks," Tusk said in a letter to EU leaders.
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 London Mayor Boris Johnson, from Cameron's party, saying he had "doubts" about their effectiveness.
UK Independence Party head Nigel Farage dismissed Tusk's proposals as "pathetic," while conservative Steve Baker said the prime minister was just "polishing poo".
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.
