Britain leaving the European Union would strip the bloc of a nuclear-armed global player and prove "disastrous" for its presence on the world stage, analysts say.

The timing could hardly be worse, as the EU struggles with its biggest migration crisis since World War II and the continent facing a growing threat from terrorism fuelled by conflict in the Middle East.

Analysts said any such division within the bloc would likely be seized upon by Russia, whose ties with the EU have been badly damaged by the Ukraine conflict.

"Great powers like the United States, China and India will see an EU weakened politically and geopolitically if there is Brexit," Vivien Pertusot, Brussels-based analyst with the French Institute of International Relations (IFRI), told AFP.

The EU has been keen to increase its influence around the world in recent years.

The bloc helped negotiate the landmark nuclear accord with Iran, and has worked closely with Washington and Moscow in an effort to revive stalled peace talks between Israel and the Palestinians.

Analysts said losing a UN Security Council permanent member and NATO lynchpin like Britain would likely diminish the EU's influence and respect around the world, while also making it more inward-looking.

- EU would lose influence -

"It would be bad news with a view to the role of the EU. It would increase the loss of image if the EU shrinks for the first time in its history," Janis Emmanouilidis, director of studies at European Policy Centre, told AFP.

"The signal would be that the EU gets slowly but steadily in a downward trend," he said, suggesting that such weakness could be exploited.

"The Chinese and the Russians might use that... to exert pressure and divide further."

Pertusot said there would also be a loss of influence in areas such as Latin America and Southeast Asia which regard the EU as a model for regional groupings such as Mercosur and ASEAN.

The prospect of a British exit has raised the possibility in some quarters that it would free up the bloc to move ahead on its own in forging a more united global position.

But analysts say there is no appetite for that, adding that most member states look to US-led NATO for security when push comes to shove in a crisis. Of the EU's 28 member states, 22 are members of the military alliance.

Rosa Balfour at the German Marshall Fund of the United States said a Brexit would effectively wreck efforts to forge what the EU now calls its Common Security and Defence Policy (CSDP).

"As a major security and military provider in Europe, a British exit... is likely to have a disastrous effect on the EU's CSDP," Balfour told AFP.

"Without British assets, it is questionable whether it is worth pursuing defence integration."

EU foreign affairs head Federica Mogherini is completing a year-long review to draw up a new policy that addresses recent changes around the bloc, such as the freeze in relations with Russia over the Ukraine crisis and the war in Syria.

The review will be submitted to EU leaders at a summit just after the June 23 Brexit vote.

- French-German plan B? -

France and Germany, the EU's historic heavyweights, would be left as the main foreign and defence players if Britain leaves.

They are said to be readying a joint Plan B in case of Brexit.

Stefani Weiss of the Bertelsmann Stiftung foundation said that Britain's influence in these areas has in fact been shrinking, with Paris and Berlin doing the heavy lifting.

"There haven't been many initiatives from the British side in terms of foreign policy recently. They did not participate in the negotiations on the conflict in Ukraine," Weiss told AFP.

Other analysts, however, doubt that France and Germany will take the lead.

They were sceptical too of suggestions Brexit might jolt the EU into action as member states realise that without Britain, they would have to step up to the plate.

France, which would be the only remaining nuclear power and UNSC permanent member if Britain leaves, is said to be more concerned about its own international role and interests than about leading a new EU security policy.

"We really do not want to imagine what would happen in the case of a Brexit," said one EU diplomatic source.