The Conservative Party's plans to charge employers for work permit "bonds" have been attacked after it emerged that the proposal would lead to upfront recruitment costs estimated at £1.2bn a year.

The plan is aimed at curbing what Michael Howard believes is uncontrolled immigration, but the Liberal Democrats yesterday warned it would impose a "huge tax" on business.

Unison, the biggest public sector union, said that the bonds would have "devastating financial consequences" for the National Health Service, which relied on skilled nurses from overseas to fill jobs in Britain.