Theresa May has warned European leaders of "damaging" consequences if the UK were to leave the EU without continuing existing security arrangements, including the sharing of information.
Speaking to a global conference of security leaders in Munich on Saturday, the prime minister called for a unique Brexit security deal to be signed as early as next year, highlighting the benefits of current data-sharing and cooperation in transporting criminals within the EU.
"In all these areas people across Europe are safer because of this cooperation, and the unique arrangements we've developed between the UK and EU institutions in recent years," she told the Munch Security Conference.
"So it is in all our interests to find ways to protect the capabilities which underpin this cooperation when the UK becomes a country outside the EU, but in a new partnership with it.
"To make this happen will require real political will on both sides."
May acknowledged that there is no existing agreement between the EU and an outside country that "captures the full depth and breadth of our existing relationship", but stressed there is "no legal or operation reason why such an agreement could not be reached in the area of internal security".
"However, if the priority in the negotiations becomes avoiding any kind of new cooperation with a country outside the EU, then this political doctrine of ideology will have damaging, real-world consequences for the security of all our people in the UK and the EU," she said.
She pointed to the European Arrest Warrant, which requires states to transfer criminals to the country that has issued the warrant. This would be abolished if a deal was not reached, she said, adding that extradition outside this system can cost four times as much and take three times as long.
"It would mean an end to the significant exchange of data and engagement through Europol," she continued, referring to the EU's law enforcement agency. "And it would mean the UK would no longer be able to secure evidence from [its] European partners quickly through the Investigation Order, with strict deadlines for gathering information requested, instead relying on slower, more cumbersome systems."
"This would damage us both and would put all our citizens at greater risk. As leaders, we cannot let that happen," she said.
In a response met with great applause, conference chair Wolfgang Ischinger told her: "It would be easier if you stayed".
"We are leaving the EU," May responded. "There's no question of a second referendum."
Jean-Claude Juncker, president of the European Commission, told reporters that security issues couldn't be negotiated in isolation from other issues.
Junker reacts to May speech: "I do not want to mix up security policy considerations with other considerations" #MSC2018