Britain supplied key information to help break up terrorist operations in four European countries in the last year, one of its intelligence chiefs revealed on Tuesday, as the UK upped the ante in the growing row over post-Brexit security.

The director of the Government Communications Headquarters (GCHQ), Jeremy Fleming, speaking on a visit to Nato headquarters, also stressed other European countries had benefited from classified intelligence shared by the UK on cyber-threats.

His comments can be seen as a direct riposte to EU chiefs threatening to exclude Britain from access to EU security databases and from Galileo, an alternative surveillance system to GPS, which was built for the US military.

It is unprecedented for a UK intelligence chief, especially one from GCHQ who until recently were seldom seen or heard in public, to intervene in a diplomatic negotiation in such a way.

Only hours earlier, Michel Barnier, the EU’s chief Brexit negotiator, in a speech in Vienna, warned that after the UK left the union it would not be involved in the European arrest warrant or the decision-making boards of Europol, or have access to EU databases after Brexit.

At the end of discussions at Nato headquarters, Fleming told reporters: “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.”

He insisted that after Brexit, the UK would continue to work with EU and EU member states, saying: “We have excellent relationships with intelligence and security agencies across the continent.”

Departure from policy

In a departure from past GCHQ policy of never discussing operational material, he made a point of how much other European countries had benefited from access to UK intelligence. “For example, we’ve played a critical role in the disruption of terrorist operations in at least four European countries in the past year. Those relationships, and our ability to work together, save lives. That will continue after Brexit, for the benefit of the UK and our partners across Europe.”

When Theresa May placed security on the table at the start of Brexit negotiations in May last year, she alarmed the heads of UK intelligence, who saw her the next day to secure a promise security would not be used as a bargaining counter. Fleming’s intervention may reflect dismay that EU chiefs such as Barnier have opted to turn it into one.

The UK shares intelligence bilaterally with France, Germany and other countries. That relationship is valued in Europe partly because GCHQ is one of the biggest surveillance agencies in the world but mainly because it is so closely intertwined with the US National Security Agency. The other two UK intelligence services, MI6 and MI5, also help counterparts elsewhere in Europe, and benefit in return.

These bilateral relationships will survive. The debate is over the UK’s continued access to specific European institutional intelligence-sharing, much of it police-related.

Fleming held discussions with the Nato secretary general, Jens Stoltenberg, the British European commissioner, Sir Julian King, and 29 ambassadors to Nato.

In his remarks to journalists, Fleming stressed the importance of GCHQ as a global leader on cybersecurity, including its shopwindow, the National Cyber Security Centre.

“We’ve worked with our European colleagues to share understanding of how to protect our democratic elections. And we’ve unmasked aggressive behaviour in cyberspace to better help businesses and citizens protect themselves. For example by joining with others to attribute NotPetya to Russia,” Fleming said.

“Over the last 12 months we will have shared classified cyber-threat intelligence with the majority of Nato member states and of course with Nato headquarters itself.”

Cyber offensive capabilities

He also dangled the prospect of sharing GCHQ’s cyber offensive capabilities. GCHQ has sophisticated cyber offensive capabilities that could be used to disrupt hostile states, international criminal gangs and terrorist groups, such as Islamic State, which the government refers to as Daesh.

“The UK government has made it clear that we’re ready to bring out offensive cyber capabilities to bear - such as those we’ve used so effectively against Daesh - for the benefit of our allies,” Fleming said.

With data-sharing one of the major areas potentially at risk from a UK-EU rift, the GCHQ director made a case for its continuation post-Brexit. No country can defend itself alone against increasingly complex global threats which “require a pooling of resource, expertise and, critically, data so that we can investigate and disrupt our adversaries”, Fleming said.

Barnier warned May that “trust does not fall from the sky”, in response to Britain’s attempt to maintain access to the EU’s security databases while leaving the institutions that provide checks and balances to prevent their misuse.

Barnier outlined some areas of future cooperation, including information exchange and involvement in Europol analytical exercises on live cases, but he insisted the status quo could not outlive the UK’s membership of the EU.

If the UK was no longer held accountable by EU institutions, including the European court of justice, it could not benefit from the bloc’s security arrangements, he said.

 “This cooperation is both unique and unprecedented,” Barnier said. “And it is made possible by the trust between member states. This trust does not fall from the sky. There is no magic wand …

“This trust is founded on an ‘ecosystem’ based on common rules and safeguards, shared decisions, joint supervision and implementation and a common court of justice. If you leave this ‘ecosystem’, you lose the benefits of this cooperation. You are a third country because you have decided to be so. And you need to build a new relationship.

“The UK has decided to leave the EU, its institutions, structures and safeguards. It will be a third country outside Schengen and outside the EU’s legal order. This is a fact. Facts have consequences.”

Barnier repeated his claim that some in the UK were seeking to blame the EU for the British government’s choices.

“Once again, we will not be drawn into this blame game. It would mean wasting time we don’t have. In this field of internal security, it is particularly hard to speak about what will no longer be possible. But we have, I have, to speak the truth.”

A spokesman for the Department for Exiting the EU said: “Keeping citizens in the UK and EU safe is an absolute priority.... We are proposing an internal security treaty to deliver this. We recognise we will be a third country, but we start from a unique position of complete alignment.

“Any drop in the breadth and quality of cooperation would have a direct impact on public safety and on our collective ability to deliver justice across Europe. Clearly this remains a matter for negotiation.”

By Ewen MacAskill and Daniel Boffey