The Government are bringing in tighter controls on illegal immigrants, and as part of this are expecting all landlords to ensure their tenants have the right to reside in the UK, and that this right remains during the whole period they are in rental property.
It is the second point that caught out the (now ex) Immigration Minister - Mark Harper. He quite legally employed a cleaner, whose right to remain in this country altered, and he didn't bother to continue checking - even though he had the might of the Home Office to check for him.
We can do no better than to quote from Shami Chakrabarti, director of Liberty, when she said it was a "bitter irony that Harper should fall foul of a mad and toxic immigration debate authored by his own government. The vile immigration bill would turn landlords and vicars into border police, checking people's status before offering them shelter or marriage services," she said. "It's the nasty immigration politics and not the politician that should go."