The penalties for this are, unfortunately, becoming ever more severe.
Until 1997 if a candidate was employed illegally, then they could be deported and would suffer a ‘black mark’ on their immigration history, but the employer faced no legal liability for having employed them.
In 1997 it became a criminal offence for you to employ someone in breach of their visa conditions.
In 1999 the law changes again. If a candidate is employed in
breach of their visa conditions they will not be subject to deportation
proceedings. Instead, they will be treated as an illegal entrant. While
deportation sounds severe, it is a procedure against which one can appeal.
Illegal entrants can be simply walked off site and onto a plane. The business
discontinuity costs of this could be very severe – particularly if you loose
several employees at once.