Half a million illegal immigrants should be given the right to stay in Britain, a think tank has said.
The Institute of Public Policy Research says such an amnesty would bring in £1bn in extra taxes, and save costs of £4.7bn needed to deport people.
It is urging Home Secretary Jacqui Smith to adopt the move, saying a large scale deportation would never happen.
The Home Office said an amnesty was unnecessary and would create "a strong pull for waves of illegal migration".
It is not known how many illegal immigrants are in the UK, with estimates varying widely from 300,000 to 900,000
The Home Office has estimated it would take more than 30 years to deport them all.
Danny Sriskandandarajah, head of migration and equalities at the Institute of Public Policy Research, said: "The simple truth is that we are not going to deport hundreds of thousands of people from the UK.
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