GCHQ played a critical role in foiling terror attacks in at least four other European countries in the last year, the head of the intelligence agency has disclosed.
Jeremy Fleming cited his organisation's involvement in disrupting terrorist activity on the continent as he highlighted the importance of UK-EU security links, saying they save lives.
Speaking after meetings at Nato's headquarters in Brussels, Mr Fleming, who was appointed director of GCHQ in March last year, said: "This visit comes at a pivotal time of course as the UK leaves the EU and as we agree a treaty on security to ensure that the UK and EU member states continue to work together to keep us all secure in the future.
"We're leaving the EU but not Europe. And after Brexit the UK will continue to work with the EU and the EU member states. We have excellent relationships with intelligence and security agencies right across the continent.
"For example, in the last year we've played a critical role in the disruption of terrorist operations in at least four European countries.
"Those relationships, and our ability to work together, save lives. That will continue after Brexit, for the benefit of the UK and for Europe."
Mr Fleming said the task of "protecting our nations and our people" is becoming more complex.
He highlighted how Daesh - also known as Islamic State - uses "slick online platforms" to "inspire and recruit a new breed of terrorists".
Criminal gangs have developed sophisticated online schemes to defraud people and businesses on a huge scale, the intelligence chief added.
He flagged up an increased threat from "aggressive foreign powers" that combine "military provocation, cyber intrusion and disinformation to impose their agendas".
Mr Fleming said: "The Russian Government has shown its blatant disregard for the consequences of its actions.
"The response to the attack in Salisbury shows this is a concern that we all share with our allies here."
He emphasised that no country can defend against the different threats alone, saying they require a "pooling of resource, expertise and critically data so that we can investigate and disrupt our adversaries".
A host of EU measures and tools have come under scrutiny following the referendum in 2016.
Senior officers have highlighted the role played by the European Arrest Warrant, a legal framework introduced to speed up the extradition of individuals between member states; the Second Generation Schengen Information System (SIS II), a database of real time alerts; Europol, the EU's law enforcement agency; and the European Criminal Records Information System (ECRIS).
While much of the co-operation between intelligence agencies takes place outside of EU arrangements, senior figures have emphasised the importance of maintaining close ties.
Last month Andrew Parker, the director general of MI5, said: "We must not risk the loss of mutual capability or weakening of collective effort across Europe."
Home Secretary Sajid Javid has also warned against any "unnecessary reduction" in co-operation.
The Government is seeking a bespoke deal on security co-operation with the EU after Brexit. A blueprint published last year called for a comprehensive framework to be underpinned by a new treaty.