An emergency brake on welfare will not stop migrants from heading to Britain, the former European Commission president has said.
Jose Manuel Barroso said proposals to reform Britain's relationship with the EU were a "creative and intelligent" compromise that "could work".
But the former prime minister of Portugal dismissed David Cameron's calls for greater British sovereignty and said restrictions on in-work benefits would fail to deter workers moving to the UK.
Britain has been "leading" the EU and the 28-nation bloc would be "weaker" if it left, he told BBC Two's Newsnight.
Asked if an emergency brake on welfare would put migrants off from coming to Britain, he replied: "No. Frankly not."
The prime minister wants new legislation to assert the sovereignty of Parliament as part of the reforms.
But Mr Barroso said in the age of globalisation, sovereignty was better protected "if we are together".
He said: "A country of 60 or even 80 million people cannot defend its rights and values in front of countries with 1.3 billion or 1.5 billion if we are alone. So, we share sovereignty."
Campaigners on both sides will step up their operations on Wednesday, with senior Tory figures Liam Fox and David Davisj joining Ukip's Nigel Farage and the DUP's Ian Paisley Jr to set out the "good life after Brexit".
Liberal Democrat leader Tim Farron is launching his party's campaign for Britain to remain in the EU and Green Party MP Caroline Lucas will join the "another Europe is possible" left-wing group pressing the case for "in".
A spokesman for Vote Leave said: "Even the EU's biggest advocate knows the PM's renegotiation is trivial. The final package won't bring powers home, won't cut the cost of Brussels and won't fix the fundamental flaws at the heart of the European Project. If the PM can't even get his own side to back the deal, why should the public? The only safe option is to Vote Leave."