North Atlantic Treaty Organisation Secretary General Jens Stoltenberg said on Monday that both the alliance and the European Union were stronger with Britain inside, and warned that a 'Brexit' could cause instability.

Stoltenberg, the former Norwegian premier who heads the 28-nation military grouping, said that it was 'up to the British people' whether they choose to remain in the European Union in a referendum on June 23.

"What really matters for NATO is I strongly believe a strong UK in a strong EU. It is good for both the UK and the EU but also for NATO," Stoltenberg told an event in Brussels held by the Politico Europe newspaper.