The alarm is raised over everything from the undermining of cross-border working to combat terrorism and organised crime to the potential boost to extremist parties across the continent.
The clock is ticking on hopes for a new deal, with the UK set to drop out of cooperation mechanisms on foreign, security and defence policy on Brexit day next March, says the Royal United Services Institute (RUSI).
Its warning comes as the Prime Minister prepares to deliver a speech to a Munich conference on Saturday, in which she will repeat her “unconditional” commitment to European security after Brexit.
However, last November, Michel Barnier, the EU’s Brexit negotiator, insisted the UK would have to leave Europol as a “logical consequence” of the vote to leave the EU.
Meanwhile, the swapping of vital intelligence information is threatened by Mrs May’s insistence that the UK will, at some point during a two-year transition period, end oversight by the European Court of Justice (ECJ).
Without a separate deal, it will take up to three years after Brexit for Britain – as a “third country” – to receive EU approval for data to be freely exchanged.
But today’s report by RUSI highlights the threat to cross-border cooperation as just one looming danger from EU withdrawal, also pointing to the risk it will:
* “Further strengthen nationalist political forces across Europe”, if a hard Brexit undermines the economic recovery in both the UK and the EU.
* Damage trade between Northern Ireland and the Republic – threatening the “fragile political settlement” in the North.
* Lead to “growing divergence” between the UK and EU’s defence and security policies – once the UK is shut out of Brussels decision-making.
* See the UK lose its leading role in European military operations, such as operational command of the EU force in Bosnia.
* Require new arrangements to support a pan-European defence industry, with the UK likely to leave the European Defence Fund (EDF).
Professor Malcolm Chalmers, RUSI’s deputy director general, said all the threats would “have costs, both financially and in relation to national security”.
“These costs can be mitigated over time, both through other existing multilateral arrangements and new bilateral mechanisms,” he said.
“However, these are real concerns, and, in combination, could risk serious negative consequences for European security.”
On terror information, Professor Chalmers called for the Prime Minister to set out whether she would “continue current levels of cooperation (for example, in relation to data-sharing), even when this involves a loss of policymaking autonomy”.
At stake is access to intelligence sharing through the Europol law enforcement agency and to the Schengen information system, which holds an 8,000-name watchlist of suspected terror suspects.
Without a deal, separate agreements would have to be struck with individual police forces and intelligence services – with the danger that vital information will “fall between the cracks”, experts have warned.
In his speech, Mr Barnier raised the fear that the UK would pursue “horse-trading” with the security of Europe’s citizens in the Brexit negotiations.
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