European Commission President Jean-Claude Juncker has warned "out is out" if British voters decide to leave the European Union.

Mr Juncker said British Prime Minister David Cameron had secured the "maximum" Brussels could give, adding there will be no further renegotiations on Britain's relationship with the EU.

His intervention on the eve of polling day there came as NATO secretary general Jens Stoltenberg played up the UK's role in boosting security, warning that a fragmented Europe will "add to instability and unpredictability".

Speaking in Brussels, Mr Juncker said: "The British policymakers and British voters have to know that there will be no kind of any renegotiation."

"We have concluded a deal with the Prime Minister, he got the maximum he could receive and we gave the maximum we could give."

"So there will be no kind of renegotiation, nor on the agreement we found in February, nor as far as any kind of treaty negotiations are concerned. Out is out."

Mr Stoltenberg praised the UK for acting as a "bridge" between the EU and NATO, plus Europe and the US, while outlining the country's role in combating terrorism and other issues.

He told The Guardian: "I don't have a vote. It's up to the people of Britain to decide."

"What I can do is tell you what matters for NATO, and 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, and a fragmented Europe will add to instability and unpredictability."

British Prime Minister David Cameron, who is urging a 'Remain' vote, says it has been a tough campaign.

Appeals from EU leaders

Italian prime minister Matteo Renzi has also appealed for Britons not to make "the wrong choice" in Thursday's vote, saying a Brexit would swap "autonomy for isolation, pride for weakness and identity for self-harm".

Meanwhile, French president Francois Hollande has said: "Tomorrow's vote in Great Britain - it's more than the future of the UK in Europe that's at stake, it's the future of the entire European Union."

"The departure of such a country that is geographically, historically, politically involved in Europe, would undoubtedly bring serious consequences."

It comes two days after the president of the European Council Donald Tusk appealed to Britain, telling voters: "Stay with us. We need you. Together we will cope with future challenges. Apart it will be more difficult."

Mr Juncker's comments drew a furious response from Vote Leave campaigner Boris Johnson, who condemned him as an "unelected tinpot figure".

Mr Johnson said: "Who elected Jean-Claude Juncker to run anything in this way? Who put him in charge of us in this way?."

"This gives the game away. If we stay in there is no prospect of any further change. This is it, folks. We have been told from the horse's mouth that any hope of further change is absolute illusion."

Polls open in Britain from 7.00am tomorrow - Irish citizens aged 18 or over who are resident in the UK are also eligible to vote.