* Barroso urges pro-EU parties to make new case for integration
* Treaty change needed to shore up euro zone
* Says structural reforms laid path to return to growth
* Expects Britain to stay in EU but sees risks
BRUSSELS, Feb 12 (Reuters) - The EU's centrist parties will retain a substantial majority in the European Parliament after May elections and must unite to re-energise the bloc, European Commission President Jose Manuel Barroso said.
Some pundits predict a group of far right eurosceptical parties including the National Front in France, Britain's UKIP and the Dutch Freedom Party, along with Greece's leftist anti-austerity party Syriza and Italy's anti-establishment 5-Star Movement, could capture 20 percent or more of the seats.
That has fuelled concern that further measures to integrate the euro single currency area could falter.
"A very clear majority will be from the pro-European parties so I don't expect major difficulties in terms of passing legislation," he said in an interview on Tuesday evening for a Reuters Euro Zone Summit with policymakers, economists and market players.
But that majority would count for little if the centre-left, centre-right, liberals and Greens did not come together and make a stronger public case for more integration which too often has been take for granted by EU politicians.
"They have to leave their comfort zone," Barroso said. "They have to make the case for Europe."
Mainstream parties should resist the temptation to ape the eurosceptics' stance against a backdrop of high unemployment, austerity fatigue and paltry growth that is fuelling support for fringe parties.
"I hope the lesson the mainstream parties draw from this is not to try to incorporate some of the arguments of the extremes," Barroso said.
The head of the EU's executive said growth was returning to Europe, albeit timidly.
Deficit-cutting measures had been crucial to assuage market pressure during the euro zone's debt crisis, he said, but far more important were structural reforms to make the bloc's economies more competitive.
Ireland, Spain and Portugal - which he expects to exit its bailout in May - had all done their homework and would reap the benefits.
"They are much more fit for global competition," Barroso said. He forecast that Europe would grow faster than expected from now on.
The Commission chief played down economists' concerns that Italy and France were falling short but acknowledged that countries which had not required bailout programmes had not felt the same sense of urgency to reform.
"They are perfectly aware and they want to do it. There is resistance ... but I believe they are basically going on the right track," he said.
Changes to the EU treaty would be required in due course to bind the euro zone closer together but that was not something to discuss in the run-up to elections, Barroso said.
"Very often the discussion in Europe is about institutions ... That's not the way to get people to come out and vote," he said. Growth, jobs and prosperity were the keys.
During the debt crisis the leaders of the United States, China, Japan and others who had called him were less interested in the details of deficit reduction than in how strongly EU member states stood behind the euro, he said.
"For those countries that share a currency it's very important to have a more integrated form of governance. This is unavoidable sooner or later.
"It's a demand of the markets that we complete our economic and monetary union."
He welcomed the German Constitutional Court's decision to refer a complaint about the European Central Bank's bond-buying programme to the European Court of Justice for a preliminary ruling, saying it showed respect for the European legal system.
THE BRITISH QUESTION
Barroso voiced surprise that Britain, long a "champion" the EU's internal market and of freedom of movement of people, goods and services, was now trying to impose restrictions.
British Prime Minister David Cameron has called for curbs on free movement and stoked concerns about migrants from Romania and Bulgaria heading to Britain in search of work or social handouts.
"You cannot have an internal market without freedom of movement," Barroso said.
Cameron is seeking to renegotiate Britain's terms of EU membership prior to holding an "In/Out" referendum in 2017.
"I hope that we can find a solution for the British concerns," Barroso said.
"I believe Britain will remain a member of the European Union. But of course when you engage in these sort of processes there are always some risks that I cannot prejudge."
He said Britain's traditional stance of advocating a liberalised internal market, free external trade, economic reform and Eastward enlargement made it important that it remained an EU member.
"I hope this same spirit is now kept and that there are not disappointments given by Britain to new member states which have always looked to it as one of their biggest supporters," he said, referring to Polish dismay at Cameron's stance.
(Additional reporting by Luke Baker, Paul Taylor and Richard Mably; Editing by Paul Taylor)