Nick Clegg said the issue of immigration was "the biggest dividing line in politics today" and branded plans for a cap "arbitrary", "pointless" and "distracting".
In an article for The Sunday Times (£), Mr Clegg wrote:
Sticking a big no-entry sign on the cliffs of Dover may be politically popular, but at a huge economic cost. What would happen if tonight every European living in the UK boarded a ship or plane and went home?
Are we really that keen to see the back of German lawyers, Dutch accountants or Finnish engineers? Do we want the NHS to fall over and the City of London to grind to a halt?
Britain would be one step closer to the exit, even though walking away from Europe would cripple the economic recovery that's been so hard won.
More top news
Khan said sorry for using the racial slur 'Uncle Toms' during a television interview in 2009.
David Cameron says the UK will hold talks with Save the Children about what more it can do for Syrian refugee children.
Rapper 50 Cent makes public apology to airport worker he accused of being "high" on drugs after finding out the teenage janitor is autistic.