Leaving the European Union would threaten Britain's security and economy, Prime Minister David Cameron said on Monday, accusing campaigners for a British exit, or Brexit, of pushing voters to accept "risk, uncertainty and a leap in the dark."
"Leaving Europe would threaten our economic and our national security," Cameron told parliament as he presented his case for Britain staying in the EU ahead of an in-out referendum on June 23.
He said cooperation could help Western states to stand up to threats such as Russia and the Islamic State, saying Russian President Vladimir Putin "likes to see disunity in the West."
"In my view this is no time to divide the West," Cameron said.
The British pound fell by its biggest margin against the dollar since 2009 on Monday, as fears of a possible British exit spooked traders.
The plunge followed a major boost to the Brexit campaign on Sunday by London mayor Boris Johnson, one of the ruling Conservative party's most popular figures, who announced his support for leaving the EU.
Europol chief Robert Wainwright, speaking in The Hague, also warned on Monday that leaving the EU would have negative consequences for Britain's security.
A vote in favour of Brexit, after more than 40 years of EU membership, would end the country's membership of Europol, the EU's police agency.
The British police depend on cooperation with the EU in the fight against terrorism and international organized crime, Wainwright said, adding that taking this away would leave a "big hole" in Britain's security.
"If you take this infrastructure away, the UK's job to protect its citizens from terror and organized crime will be harder," he said.