British Prime Minister Theresa May has made her case for a new security treaty with the European Union from next year, winning support from EU and US officials who agreed the issue was too important to risk getting subsumed in broader Brexit negotiations.
In a speech to western leaders and officials in Munich, Mrs May promised that London would continue to lead military missions and share intelligence if Brussels agreed to a pact "effective from 2019", the year Britain is due to leave the bloc.
Mrs May's government is using a series of speeches to set out its vision for Britain outside the European Union.
But the loudest applause during her appearance came when the event's organiser, German diplomat Wolfgang Ischinger, said: "Things would be so much easier if you stayed."
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But Mrs May was adamant: "We are leaving the EU and there is no question of a second referendum or going back, and I think that's important," she told the Munich Security Conference.
"The partnership that we need to create is one that offers UK and EU way to combine our efforts to greatest effect where this is in our shared interest," Mrs May said of her security plan.
Britain is one of the top three users of data from European Union police agency Europol. But as it leaves the EU, there is a risk that it will be shut out of this co-operation, becoming more vulnerable to Islamist militants, officials say.
Britain, along with France, is Europe's biggest military power and leads two EU military missions while sending troops to Estonia under a NATO flag.
With a host of issues still unresolved and infighting over Brexit dividing Mrs May's government just over a year before Britain is due to leave, security is one of London's biggest bargaining chips as it seeks a new deal with Brussels.
Britain's interior minister last year told the EU it could "take our information with us" if it left the bloc without a deal on security.
Mrs May emphasised she was committed to European security, warned against competition between Britain and the rest of Europe and said that both sides should do "whatever is most practical and pragmatic in ensuring our collective security".
Mr Juncker said security should not be conflated with "other questions relating to Brexit," also quashing any British hopes that a security treaty might be a way into a free-trade deal.
"I wouldn't like to put security policy considerations with trade policy considerations in one hat. I understand why some would like to do that, but we don't want to"
NATO Secretary-General Jens Stoltenberg also welcomed close security cooperation between Britain and the EU after Brexit, while Mr Stoltenberg's predecessor at the alliance, Anders Fogh Rasmussen, said Mrs May's speech was strong.
"I think that is what we should aim for, but any treaty is very time-consuming. The timeline is unrealistic," Mr Fogh Rasmussen said.
A senior US official said: "We know the goal, but I don't know if this is the right vehicle."
