EU faces big changes, with or without UK

The atmosphere in Brussels has been surreal for weeks -- business goes on as usual, officials are barred from using the word "Brexit" and life proceeds as if Britain's EU referendum does not exist.

But this phoney calm belies the fact that, whether Britons vote to leave or remain, the European Union will have to make some of the biggest changes in its history in order to survive.

If Britain becomes the first member state to leave the EU, it could trigger the beginning of the end for a beleaguered union mired in a migration crisis, economic woes and a growing threat of terrorism.

Even if it stays, the status quo will not be an option, with the questions raised by Britain's referendum reverberating around a continent that is losing faith in the post-war European dream.

"Whatever its result is going to be, we must take a long hard look at the future of the union. We would be foolish if we ignored such a warning signal as the UK referendum," EU President Donald Tusk warned this week.

A British exit will plunge the EU into years of bitter divorce negotiations between Brussels and London, although in some quarters at least there are hopes it will at least let the rest of the bloc get on with its work.

That may be easier said than done.

If Britain remains in the EU, Prime Minister David Cameron has already warned that he will return to Brussels to push for more reform on freedom of movement on top of the renegotiation deal that he secured in February.

Free movement is a core EU value and Britain will face severe opposition. More likely are changes to the rules of the Schengen passport-free area, which has already been semi-dismantled by Europe's migration crisis.

The fear in many European capitals is that either way, the result could trigger a domino effect of referendums in other countries.

French far-right leader Marine Le Pen on Tuesday urged all EU states to follow Britain's example, and eurosceptics in the Netherlands, Denmark and Sweden have made similar calls for referendums.

Tusk has warned that a British leave vote could lead to the "destruction of not only the EU but also of Western political civilisation."

Bickerton said it would unlikely be a "terminal blow", given the core role of the EU in much of European political life, but that it would herald a fundamental change and a move towards a far looser kind of union.

"I don't think it would suddenly disappear but over the longer term we might see it slowly decline and become something different," he said.

"The EU is in a negative spiral."