The head of NATO says it is essential for Britain to remain in the European Union to effectively fight terrorism.

"A strong UK in a strong Europe is good for the UK and it's good for NATO, because we are faced with unprecedented security challenges, with terrorism, with instability and an unpredictable security environment," Jens Stoltenberg, NATO's secretary general, told The Guardian.

"And a fragmented Europe will add to instability and unpredictability."

Stoltenberg's remarks come as Britain prepares for Thursday's historic "Brexit" vote, in which its citizens will decide whether to quit the European Union.

He noted that Britain is the "biggest force provider" among European NATO allies.

"So it matters what the UK does and for NATO it is an advantage to have UK leadership inside the European Union being a strong advocate for transatlantic cooperation and also for EU-NATO cooperation," Stoltenberg said.

Up-to-the-minute polls suggest that the vote is too close to call.

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