An employer who follows these steps will not be prosecuted if he or she is found to be unknowingly employing an illegal immigrant. Employers who deliberately employ illegal workers and use forged documents as a defence will, in theory, be prosecuted.

Tom Moran, of the CBI, said: “It’s a real challenge for employers to check that employees are here legally. In some cases they (employees) have used forged documentation but have a genuine national insurance number. Even those employers who are doing their best are tricked by high-quality forgeries.”

He said that it was difficult for employers to keep up with the changing rules and urged the Government not to go after those who unwittingly employed illegal immigrants.

However, he acknowledged that rogue companies did ignore the rules and turn a blind eye to fake documents. “The Government should be looking to throw the book at these people,” Mr Moran said. “We need targeted enforcement of those who are outside the system.”

Stephen Alambritis, of the Federation of Small Businesses, said that its members had to check up to 80 documents to ensure that workers were legal.

The Refugee Council said last night that Britain would benefit if those working illegally were allowed to work legitimately. It pointed to a report by the Institute for Public Policy Research which said that the Exchequer could accrue £6 billion by decriminalising those working illegally in Britain.

The charity, which represents asylum-seekers rather than economic migrants, said: “Most of them are very desperate to work and support themselves, because they don’t want to live off state benefits.” It was surely better that people should pay tax and national insurance rather than being possibly tempted to be exploited in the black economy, the Refugee Council added.