Is it the responsibility of the employer to require existing employees to produce documentation relating to their immigration status?

Under the Immigration, Asylum and Nationality Act 2006, employers are, in some circumstances, obliged to check whether existing employees continue to have the right to work in the UK. This is the case where a worker started working for the employer on or after 29 February 2008 and where his or her right to remain and work in the UK has been granted for a limited period only.