The Unite union has said it is seeking legal advice about whether the Home Office “incited racial hatred” by sending vans around London encouraging illegal immigrants to “go home”.
Its leader, Len McCluskey, called the poster-covered vehicles “vans of hate”.
Nick Clegg has criticised the pilot scheme, but the Home Office has said the message was not racist.
Meanwhile, new peer Doreen Lawrence has said police stop-and-check operations focus “mainly on people of colour”.
The Labour-supporting mother of murdered teenager Stephen Lawrence, told ITV’s Daybreak she thought “racial profiling” was involved – a suggestion rejected by ministers.
The controversy comes after it was revealed that nearly 140 people had been arrested in a wave of raids aimed at tackling people working illegally in the UK and firms employing them.