A MASS amnesty of illegal immigrants would cost Britain £4billion a year, campaigners have warned.

London Mayor Boris Johnson is urging the Government to give up deportation plans for 700,000 illegals working in Britain.

The top Tory reckons we would be better off letting them stay because it will “boost” tax revenues as they start paying into the system.

But immigration campaigners last night said the idea was “ludicrous”. They say taxpayers will have to fork out £80million a week in benefit and housing bills to fund illegals.

Sir Andrew Green, chairman of Migrationwatch UK, said: “Clearly the British public can see that to reward people with a meal ticket for life for breaking our laws is an absurd proposition.

“It will only encourage others to come and take their place in the hope of a further amnesty.

“That is exactly what has happened in Italy which has granted five amnesties in the past 20 years and Spain which has granted six.

“Claims of ‘a huge increase in tax revenues’ are ludicrous when benefit payments are taken into account.

"The real victims are British workers whose wages are held down or perhaps their job opportunities taken by people who came or remained here illegally.”

Shadow Home Secretary Dominic Grieve warned it would be “massively counter-productive”.