If Tony Blair can reform the EU then the UKIP might back British membership, UKIP's leader in the European Parliament has told the prime minister.
Mr Farage issued a challenge to Mr Blair
Nigel Farage said Mr Blair's call to cut EU red tape and reconnect with voters "sounds like the sort of thing UKIP has been saying for 10 years".
And he dubbed Mr Blair "a Europhile who has been mugged by reality".
Mr Blair, in Brussels to address MEPs, hit back, saying he wanted to "reinvigorate not wreck" the EU.
"I don't want Britain to be in a position of leading the charge against the EU," Mr Blair said.
Earlier, in a speech to the Brussels parliament, Mr Blair told MEPs the EU faces a "crisis in political leadership" and must change to win back public support.
He said failure to modernise the EU and reconnect with people's concerns could be exploited by those peddling "outdated nationalism and xenophobia".
"Ideals survive through change, they die through inertia in the face of challenge," he told MEPs.
Mr Blair said he was far from Eurosceptic, telling MEPs: "I am a passionate pro-European. I always have been,"
He said he would "never accept" a Europe that was simply an economic market," adding "we need a social Europe, but it has to be a social Europe that works."
Mr Blair takes over the six month rotating presidency of the European Council next month.
UKIP, which got nearly one in six British votes in the last European election, has campaigned for British withdrawal from the EU.
Mr Farage told MEPs Mr Blair was the "only European leader who understands why France and Holland voted no" to the European Constitution.
He said UKIP wanted a return to a "common market" promoting "free trade across Europe".
He said he accepted Mr Blair could not do that in six months, but he said the "challenge" of his presidency was to turn back the growth EU laws that were stifling small and medium-sized business.
"How are you going to turn that ship around?", asked Mr Farage.
"If you can reform the European Union, Mr Blair, then I may even change my mind, I may even think it's worth us staying a member," he added.