"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 on Monday.

Stoltenberg, the former Norwegian premier, said that it was "up to the British people" to decide about the future of their country.

Britain will vote on June 23 on whether or not it should stay in the EU. The decision has far-reaching consequences for both the country and the bloc.

"It is a great advantage to have the UK being strong in the EU, pushing for security cooperation with NATO... It's good to have the UK in NATO pushing for strategic cooperation with the EU," said Stoltenberg who heads the 28-nation military grouping. "We live at a time of many threats and instability, we don't need more instability, we need more cooperation in Europe.

British Prime Minister David Cameron (L) speaks with NATO Secretary-General Jens Stoltenberg during the European Union leaders summit in Brussels on June 26, 2015.

Both the EU and NATO have so far avoided intervening in Britain's referendum debate, in which Prime Minister David Cameron is trying hard to prevent Brexit.

Cameron and most of his cabinet are sparing no effort to convince Britons to vote against leaving the EU, but about half of his MPs support an exit.

The prime minister has consistently warned against Brexit, saying that it poses an economic risk to the country and can undermine Britain’s security.

However, according to former Mayor of London Boris Johnson, nearly 300,000 new jobs will be created if Britain escapes from the EU "shackles."

“If we Vote Leave, we will be able to forge bold new trade deals with growing economies around the world. These are deals that the EU has tried and failed to achieve due to protectionist forces in Europe,” said Johnson at a Brexit campaign rally in London on Saturday.

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