UK citizens could be more vulnerable to global terror attacks if Britain withholds security co-operation from EU nations who back a Brexit extension, according to a senior European politician.

Neale Richmond, the Irish Senate’s Brexit spokesman and a close ally to Taoiseach Leo Varadkar, delivered the warning after a Downing Street official made what he described as an “odious” threat.

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In a lengthy anonymous briefing to The Spectator, the official – named by Tory MP Amber Rudd as likely to be the Prime Minister’s top aide Dominic Cummings – suggested Number 10 could punish the remaining 27 EU member states if they support extending Article 50 beyond October 31.

The official said: “We will make clear privately and publicly that countries which oppose delay will go the front of the queue for future co-operation – co-operation on things both within and outside EU competences.

“Those who support delay will go to the bottom of the queue.”

The article said the aide “also made clear that defence and security co-operation will inevitably be affected if the EU tries to keep Britain in against the will of its government”.

Richmond hit back, pointing to the nerve agent attack in Salisbury last year which exposed UK vulnerabilities to global terrorism.

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“Certainly the suggestion in the leaked missive from an unnamed Downing Street source that the UK would withhold security co-operation to EU countries is extremely worrying and downright odious,” he said.

“At a time when global terrorism remains a threat with the increasing focus on cyber security such a notion is particularly strange given the UK’s very recent experience in terms of the events in Salisbury.”

The National: Neale RichmondNeale Richmond

The threat made by the No 10 official was condemned by Northern Ireland Secretary Julian Smith.

Speaking after a Cabinet meeting yesterday morning, Smith said: “I am clear that any threat on withdrawing security co-operation with Ireland is unacceptable. This is not in the interest of Northern Ireland or the Union.”

Richmond said he was “relieved” to hear the idea was not backed by Smith as it would “jeopardise decades of close co-operation between security forces” on both sides of the border.

“Within the context of Ireland, such a move would jeopardise decades of close co-operation between security forces dealing with gangland crime, people smuggling and of course the dissident paramilitary threat,” he said.

“I am relieved, however, to see the intervention by the Secretary of State for Northern Ireland on Twitter who explicitly stated that any such threat would be unacceptable.”

Meanwhile the Prime Minister’s official spokesperson also distanced Boris Johnson from the idea.

“The Prime Minister has set out clearly on any number of occasions that we continue to have a close security partnership with our EU colleagues,” they said.

Earlier, a Number 10 source did not deny Cummings was behind the briefing to The Spectator, saying the comments were “the strong view of a senior Number 10 official”.

Rudd, the former Tory Cabinet minister, branded the comments “angry and desperate”, and said the lack of denial from Number 10 meant she could “only assume it’s come from the centre, from the Prime Minister’s adviser, and the style of it seems to imply that”.

Rudd, who lost the Conservative whip for helping MPs pass a law aimed at thwarting a no-deal Brexit at the end of October, added: “I think it’s Dominic Cummings, otherwise it would have been heavily denied and heads would have rolled.”

As a member of the EU, the UK co-operates on security and identifying terror risks with the other nations.

As a member of Europol the UK has access to resources including the Schengen Information System (SIS), European Arrest Warrant (EAW) and European Criminal Records Information Exchange System.

No country that is not a member of the EU or Schengen zone currently has access to the SIS or is party to the EAW, which generates more than 2000 arrests every year in Britain.

Last year former Scottish justice secretary Michael Matheson said police and prosecutors could face “complex and far-reaching” challenges in tackling cross-border crime post-Brexit.

He said: “From cybercrime, to human trafficking and terrorism – international crime has never respected borders. Over the years Scotland’s police and prosecution services have built strong links with their EU counterparts to help keep people safe,” he said.

“Withdrawal from the current regime of co-operation, including for example the European Arrest Warrant system, could mean returning to a more fragmented system of seeking assistance across borders.

“We risk being left behind as our European counterparts develop more effective tools to deal with present and future threats.”

In the last five years 70 extraditions to Scotland and 361 extraditions from Scotland were carried out under the European Arrest Warrant system.

Responding to the briefing, the Green Party’s Caroline Lucas described the government’s position as “as close to blackmail as it gets”.