“Leaving Europe would threaten our economic and our national security,” Cameron said in a report to parliament after dozens of MPs from his own Conservative party said they will vote for Brexit, including London mayor Boris Johnson.
Cameron has called for an in-out referendum on Britain’s EU membership after securing a deal on reforms at a summit in Brussels last week.
“When it comes to people’s jobs it is simply not enough to say it will be all right on the night and we will work it out,” he told parliament.
“We need to properly face up to the economic consequences of a choice to leave,” he said, warning that it would be “a great leap into the unknown.”
Cameron also warned about global threats from Russia and the Islamic State group, saying: “The challenges facing the West today are genuinely threatening …. In my view this is no time to divide the West.”
In a barb at London mayor and Conservative party rival Boris Johnson, who declared on Sunday that he would vote to leave the EU, Cameron said: “I have no other agenda than what is best for our country.”
Johnson, who is seen as a potential successor to Cameron as Conservative leader and prime minister, has been accused of pursuing political ambitions with his decision.