Brexit could worsen crime in the UK and damage security across Europe, a leader of the EU’s law enforcement agency has warned.

Brian Donald, Europol’s chief of staff, said that although the Government has drawn up proposals to remain a part of the organisation an “adverse impact” should be expected.

Theresa May herself warned of the security threat posed by leaving the EU during the referendum campaign, when she was the serving Home Secretary.

Since becoming Prime Minister she has supported the current Government’s position that any risk will be mitigated by new agreements, despite concerns raised by opposition parties and experts including security commissioner Sir Julian King.

Speaking at a summit in London, Mr Donald said Brexit’s effect on both UK and EU security depends on the outcome of negotiations and the final exit deal.

But David Davis and Michel Barnier have not yet started discussions on the issue, which are due to take place in the delayed “second phase” of floundering talks.

“Almost certainly the arrangements governing the UK’s police cooperation with EU partners will not be as deep and effective as they are today,” Mr Donald warned. “So this process is about minimising adverse impact.”

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The former Serious Organised Crime Agency officer said Brexit would also have an impact on crime in the UK, especially if it loses 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 2,000 arrests every year in Britain.

Mr Donald told the National Security Summit that serious criminals could exploit resulting intelligence gaps that would make them harder to trace, adding that any curbs on freedom of movement will generate business for people smugglers and economic de-regulation would create a “honeypot for fraudsters”.

“Reasonable assumptions point to a worse situation than now in the UK,” he added.

“I am sure the British authorities are studying these scenarios closely but of course any change in crime trends will very much depend on which Brexit we end up with.”

Mr Donald said Europol had repeated calls for security cooperation to be “among the highest priorities in the Brexit negotiations” in the interest of both sides amid heightened security threats and terror attacks.

The Government has proposed a new security treaty between the UK and EU to ensure a “comprehensive new security, law enforcement and criminal justice partnership” after Brexit.

A policy paper published last month, days after the attempted bombing of a London Underground train, said the treaty would “would be underpinned by our shared principles and should make sure our partnership has the agility to respond to the ever-changing threats we face”.

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Mr Davis, the Brexit Secretary, said at the time: “We already have a deep level of collaboration with the EU on security matters and it is in both our interests to find ways to maintain it.”

The Home Office said the UK had been a leading contributor to information sharing and in the EU level.

Europol already has a model for cooperation with non-EU states, including the US, Australia and Norway, but any negotiations over continued British membership will be handled by the European Commission.

It is expected to impose conditions on any agreement, such as on the UK’s data protection enforcement, or remaining under the European Court of Justice’s jurisdiction.

Britain will retain its current security arrangements with other international bodies linked to the G7, Interpol and the United Nations.

A Home Office spokesperson said: “As we prepare to leave the EU it is vital that we agree a new way to ensure continued security, law enforcement and criminal justice cooperation.

“This is why last month the Government outlined its proposal to seek a new treaty with the EU which will underpin our future partnership, building on the already deep level of collaboration we have.