The UK's access to some systems it depends on to keep its people and borders safe could be diminished after Brexit, the head of the EU's law enforcement agency has said.
Rob Wainwright suggested that redrawing arrangements around police cooperation tools following a vote to leave may take at least two years.
The Welsh-born director of Europol told the Press Association the need for international police cooperation is "stronger than ever" in the face of terrorism and serious crime.
A highly-charged debate on security has exposed divisions at the top of government and within the intelligence community ahead of next month's referendum.
While some prominent figures have warned about the repercussions of leaving, others have played down the risk and suggested there could be benefits.
Mr Wainwright said threats are now "much more complex and international in nature".
He said: "Drugs are coming from abroad, terrorists are radicalised in Syria and Iraq and coming back.
"Our real security challenges are European in nature so we need to stay strong together to face them together."
EU instruments provide British authorities with "vital information" every day, he said, adding: "If you take that away at this critical time ... then of course you weaken your overall defence mechanisms in the face of those threats."
The UK "would lose some of its access to some of the systems that it currently depends on to keep its borders and its people safe" if it departed, Mr Wainwright claimed.
He continued: "How you mitigate that risk depends on how the negotiations would go afterwards.
"At the moment Britain's security relies on a number of things, which includes access to EU information systems.
"If you put that access under any kind of threat or diminish that access it's bound to have potential negative consequences."
Hundreds of messages are exchanged through Europol channels between British authorities and their European counterparts every day, he said, while 2,500 new cases for cross-border co-operation were launched through the body last year.
Other tools include the European Arrest Warrant and the Schengen Information System database.
Mr Wainwright said: "I'm pretty sure because Britain is such a strong partner that it would get a pretty good deal in having at least partial access to most of those systems."
However, there is a "lot of uncertainty" about any deal, he said.
"I'm not saying the sky is going to fall in," he said. "All I'm saying is that we have to accept that EU systems form an instrumental part of the way we protect ourselves these days and if leave we are not going to have the same effective access to those as we have now."
On how long it could take to form new agreements after Brexit, he said Britain is currently participating in around 35 instruments on security, adding: "It's going to take some years. I guess at least two years, maybe longer."
Creating a parallel system for the UK alone would be a "huge logistical exercise", Mr Wainwright said.
Asked about any suggestion of scaremongering over warnings relating to security, he said he was not "part of a political process" adding: "I'm not campaigning for any political party or either side and neither are my fellow colleagues.
"None of them - not one of them who is a current chief that I've spoken to - believes that our security wouldn't be affected if we left."
The Paris and Brussels attacks showed information-sharing levels were "still not great" across Europe, Mr Wainwright conceded.
He added: "Europe hasn't got this perfectly right yet, but what's our response to that – to walk away from that or stay in and continue to improve? I think it has to be the latter."
Home Secretary Theresa May has said the country would be more secure from crime and terrorism if it remains a member of the EU.
However, there have been contrasting arguments from former intelligence chiefs and other cabinet members.
Former head of MI6 Sir Richard Dearlove argued there were potential security gains from Brexit, while Justice Secretary Michael Gove has dismissed warnings about the potential dangers.