A former MI6 chief has said Brexit negotiations have got to “an extraordinarily damaging state” because unnamed Downing Street officials have been briefing that the UK might withdraw security cooperation with the EU.

Sir John Sawers, in a briefing to reporters at a technology summit, said any attempt to stop working with EU countries in tackling terrorism “would be an act contrary to our own self-interest”.

The one-time spy boss, who headed MI6, the foreign intelligence service, between 2009 and 2014, described Europe as “our defence in depth” and argued that friendly countries gained considerably from sharing intelligence on terrorism.

“It’s a team game, where you gain more by pooling that information. And by sharing your knowledge and your understanding of what’s happening,” Sawers said, adding that helping European countries denied terrorists a wider space in which they might otherwise operate.

On Monday evening, the Spectator published an incendiary 700-word memo from a Downing Street staffer on the state of the Brexit negotiations, which contained a warning that the UK might halt security cooperation with the EU if Brexit was not achieved by Boris Johnson’s deadline of 31 October.

Downing Street’s tough-talking ‘memo’ is part of a dangerous game | Gaby Hinsliff Read more

The author of the memo, sent by text, is not known, although many suspect it is the prime minister’s chief adviser, Dominic Cummings.

Sawers said he recognised the UK would not share “vital defence with a hostile power” such as Russia or China, but said it would be extraordinary if relations between the UK and the EU collapsed to the point where security cooperation were to be withdrawn.

“Things would have got to a very dangerous pass if they deteriorated that far,” he added.

Jihadist attacks have taken place in a number of European countries in recent years, with the UK, France and Belgium particularly hit; while Ireland and the UK cooperate on dealing with terror threats relating to Northern Ireland. Sharing intelligence and information about threats and methods is routine among western nations in order to maintain security.

Sawers’ warning, delivered at the Digital Transformation EXPO Europe conference, comes a day after cabinet ministers expressed their unease over the threats. Julian Smith, the Northern Ireland secretary, said: “I am clear that any threat on withdrawing security cooperation with Ireland is unacceptable. This is not in the interest of NI or the union.”

The exact wording of the threat made by the author of the memo was not precisely spelled out in the Spectator. But the memo says: “We will make clear privately and publicly that countries which oppose delay will go the front of the queue for future cooperation – cooperation on things both within and outside EU competences.

“Those who support delay will go to the bottom of the queue. [This source also made clear that defence and security cooperation will inevitably be affected if the EU tries to keep Britain in against the will of its government].”

Sawers highlighted that the UK would lose access to airline passenger name records and the ability to use European arrest warrants in the event of a no-deal Brexit. Police have also warned that there would no longer be fast access to intelligence in the Schengen Information System II database.

The former spy chief added he was concerned about the impact of a no-deal Brexit in security terms, saying the withdrawal agreement negotiated by Theresa May but rejected repeatedly by parliament contained provisions that would allow the UK and Europe to share “a single security space”.

He warned that otherwise the UK would have to “start with a blank sheet of paper and have to start all over again. And the UK would would not have the automatic right of have access to those facilities. That will be a loss in itself.”