Being in the EU makes it harder to stop serious criminals and those with suspected terror links entering the UK, a government minister says.
Dominic Raab, who backs EU exit, said EU nationals whose activities are of concern but about whom there is no clear intelligence have a "free pass" into the UK due to free movement rules.
Debate on security within the EU has intensified since the Brussels attacks.
Pro-EU campaigners say the UK will lose access to key information by leaving.
Prime Minister David Cameron has insisted co-operation among 28 states makes the UK safer while former security service officials have expressed contrasting views about the importance of EU membership with regard to intelligence sharing arrangements with the US and other allies.
Mr Raab's intervention, in a speech in London, marks a continued focus by Leave campaigners on the issue of security and what they argue are the risks of EU membership.
On Tuesday, those calling for an Out vote in the 23 June referendum released a list of 50 foreign criminals they say have been allowed into the UK because of freedom of movement rules giving EU nationals visa-free entry.
Speaking to BBC Radio 4's Today programme ahead of his speech, Mr Raab said the UK had been able to stop only 6,000 EU citizens from entering the UK, compared to "67,000" from outside the EU.
He said this was because EU rules meant EU nationals could only be barred if they presented "a genuine, serious and current threat". Criminal convictions alone were not enough to block entry, he said.
Mr Raab said this was "a very serious chink in our security apparatus and it is a direct result of EU".
In his speech the justice minister argued that the sheer number of people who could legally come to the UK from the EU made it "exponentially harder" for those presenting a "credible and current danger" to be monitored and stopped.
Although the UK is not a member of the Schengen borderless travel zone and carries out passport checks, Mr Raab said the current system reduces the capacity for effective surveillance and leaves the UK effectively "importing risk".
"Crucially, for UK intelligence agencies, we cannot bar individuals on whom we have sketchy intelligence but reason to believe may be linked to terrorist related or other serious criminal activity. Or who may have done something which gives rise to questions, such as visiting Syria, without a clear or credible reason," he said.
He added: "EU rules set the bar for taking meaningful action impossibly high, which means we effectively have to give a free pass into Britain to those coming from the EU."
'Spirit of Wembley'
UKIP leader Nigel Farage caused controversy in the immediate aftermath of the Brussels attacks by claiming the city had become the "jihadi capital of Europe" and EU border rules led to "the free movement of terrorists, of criminal gangs and of Kalashnikovs".
Mr Raab stopped short of saying this, arguing it was "too early" to tell to what extent Schengen rules helped those behind the attacks, but he insisted "regaining control over our borders would be a valuable defensive tool in protecting Britain from future terrorist attacks".
Former shadow home secretary Yvette Cooper, part of the pro-EU campaign group Britain Stronger in Europe, said it was "desperate, false and irresponsible stuff" from the leave campaign.
She said the UK was outside Schengen and "can operate full borders and security checks", while the home secretary had the power to bar any foreign national considered "a threat".
Leaving the EU would make cross-border co-operation to fight crime and terrorism "so much harder", she added.
Meanwhile, Mr Cameron, who will return to the EU campaign trail next week after the Easter recess, has been warned that he will only win the referendum by putting forward a "positive and inclusive" vision of the benefits of the EU membership.
A group of peers has said the PM must focus on articulating the shared values that different European countries have in common.
The House of Lords EU committee said relying on "narrow national economic self-interest, alongside fear of the alternatives" would not be enough to ensure an In vote in three months' time.
"It needs to try and capture the spirit we saw in Wembley last year when England football fans sang the Marseillaise after the attacks in Paris," said Lord Boswell, the former Tory MP who chairs the committee.