"I see a very clear picture of the United Kingdom's dependency on the EU to help protect its security interest," Europol's director Rob Wainwright said in The Hague.
Should Britain leave in a so-called "Brexit" it will "no longer have the benefits that it currently has," Wainwright told reporters in The Hague, speaking on the sidelines of a conference on combatting migrant smugglers into Europe.
This included "direct access to our database, the ability to involve itself into our intelligence projects and many other areas," he said.
Britain would be "safer, stronger and better off" in the 28-member bloc, Cameron said Saturday after announcing June 23 as the date for a referendum on the issue.
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The issue has deeply divided Britain's ruling Conservative Party with five cabinet members as well as London's outspoken mayor Boris Johnson supporting the "Leave" campaign, and the country's continued security is a key issue.
Following Cameron's remarks, Work and Pensions Secretary Iain Duncan Smith told the BBC that staying in the EU would make the country more vulnerable to Paris-style attacks.
Wainwright however said even if Britain would negotiate an agreement with Europol in the event of leaving "it will not be a full member any longer and will not enjoy the benefits.