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
An Indian soldier has been found alive six days after an avalanche hit the Siachen glacier in the Himalayas.
A cruise ship had to abandon its voyage to the Bahamas after being caught up in hurricane-like conditions off the US coast at the weekend.
A cold and perhaps frosty start, with a risk of wintry showers in the south west. Sunny spells and showers for the rest of today.