Brexit threatens Britain’s security unless it wakes up to the fact it must make concrete demands in the negotiations and stop assuming “good intentions” will suffice, experts have warned.

Though both Britain and the EU have emphasised they want to continue cooperating closely, a report by The UK in a Changing Europe warns that the matter is so “fiendishly difficult” that a new “cliff edge” on the issue looms unless Britain is cleared about what it wants.

“There is a danger that, unless the British Government acts quickly to define more clearly what it wants and how it might achieve it, another Brexit cliff edge – in security –  might be on the horizon,” Professor Anand Menon, King’s College London academic and director of the Brexit-focused research body, said.

Britain has been accused of using security, one of its stronger suits in the negotiations, as a bargaining chip to ensure it gets a better economic deal.

Menon added: “This is fiendishly complex. When negotiations are likely to involve constitutional issues, disagreements over the role of the ECJ and trade-offs from both sides, good intentions are not enough.

“Despite a shared desire to cooperate closely in future, nothing can be taken for granted.”

The UK in a Changing Europe report, published on Friday, argues British negotiators have failed to lay out specific enough demands on issues such as the European Arrest Warrant (EAW), participation in Europol and intelligence sharing between police forces and could lose out amid trade-offs.

It warns that any deal on the EAW would likely take years to negotiate and, while nations like Iceland and Norway have negotiated their own deals, the end result for Britain would likely be some EU countries wouldn’t surrender their nationals to the UK.

Britain is an active participant in Europol but it may any operational role in the agency, unless it can negotiate a new relationship that is “unprecedented”, the report said.

The Government has sought to emphasise the importance of security but also to deny it was trying to “blackmail” the EU by emphasising this in public.

Theresa May was accused of making a “blatant threat” when she said security could be weakened if Britain left the EU without a deal in her Article 50 letter in March.

“I think the security of our citizens is far too important to start a trade-off of one and the other. Both are absolutely necessary in the future partnership without bargaining this one against the other,” European Parliament’s Brexit co-ordinator Guy Verhofstadt said in response.

In September, the Government issued its position paper, noting “belief the UK has a historic deep belief in the same values that Europe stands for – peace, democracy, freedom and the rule of law” and making no reference to any threat of withdrawing co-operation.

Then-Defence Secretary Michael Fallon told the BBC: “This isn’t blackmail, this isn’t a negotiating strategy. What we are doing, and everybody has asked for this, is to set out how we see the new partnership the day after Brexit.

“We want to fight terrorism together. It’s vital. We are not making threats.”