Britain will be less safe from international criminals, would be diminished on the world stage and suffer economically if it votes to leave the European Union, the campaign to retain ties with Brussels has warned.
At a launch event for the Britain Stronger In Europe campaign, former Association of Chief Police Officers president Sir Hugh Orde warned that "villains" would come to the UK to escape justice if the country pulled out of EU-wide agreements on extradition and sharing evidence.
The In campaign's figurehead, businessman Lord Rose, warned that a vote to leave the EU would be a "leap into the dark", putting the country's prosperity and security at risk.
Sir Hugh, the former chief constable of the Police Service of Northern Ireland, put the dangers of a British exit in stark terms.
Speaking at the launch event in east London, he said: "If I was a villain somewhere else in Europe and I was escaping justice, I would be coming here because it's going to take a lot longer to get you back."
Former Marks and Spencer boss Lord Rose warned jobs would be put at risk and prices could rise if the country voted to leave the EU in the referendum promised by David Cameron by the end of 2017.
He said: "Those who want us to leave Europe would risk our prosperity, they would, I believe, threaten our safety and they would, I believe, diminish our influence in the world.
"We know our economy would take a hit, what we don't know is how bad that hit would be.
"They have no idea whether we would be able to access Europe's free trade area, or what the price of admission would be.
"They have no idea how long it would take to renegotiate existing trade deals or how difficult it would be to negotiate new ones."
He continued: "They cannot guarantee that jobs would be safe or that prices wouldn't rise.
"They cannot explain how we would stop free movement and simultaneously keep access to the world's largest duty free market.
"They cannot say how our diminished status would impact on our relationships with the US or with China or, indeed, with Commonwealth countries.
"Leaving Europe is a leap into the dark and I don't believe that is a risk that is worth us taking."
Lord Rose insisted the campaign was not claiming the UK could not survive outside the EU but questioned whether the country could thrive.
"It's not about patriotism, it's about pragmatism in my view," he said.
He cited Confederation of British Industry (CBI) estimates that the benefits of trade, investment, jobs and lower prices mean membership is worth £3,000 per year on average to every UK household.
Based on an annual contribution to Brussels of around £340 per household, there was a "10-to-one return" for Britons, he claimed.