It would cost the UK £4.7bn to deport all illegal immigrants
Granting amnesty to long-term illegal immigrants in the UK, could add up to £3bn to the economy, a report has said.
The London School of Economics report said the move would not lead to a rise in migration but would raise spending on welfare services and housing.
About 618,000 people are illegally in the UK, of which 442,000 are in London.
London Mayor Boris Johnson commissioned the study and said it proved immigrants were "far from a financial burden" and had highlighted "long-overdue facts".
The study found that if a five-year residency plan was introduced 67% of illegal immigrants would be eligible to live in the UK.
Mr Johnson said: "This new report has introduced some long-overdue facts, hard evidence and academic rigour into a debate which has far too often been dominated by myth, anecdote and hearsay.
"So, far from a financial burden, as some suggest, this new research has found an amnesty could be worth up to £3bn a year to the country's economy.
"The study also demolishes the argument that an amnesty would inevitably lead to increased migration to the UK and identifies effective border controls as the vital factor in controlling and deterring illegal immigration."
Mayoral policy director Anthony Browne says the amnesty's cost could be contained
London Assembly chairman Darren Johnson welcomed the report's publication.
He said: "This report lends further economic weight to the argument in favour of regularising the status of hundreds of thousands of Londoners who are already making a significant contribution to the capital's wealth."
Previously, Immigration Minister Phil Woolas had said the mayor's call for amnesty was "naive" and would lead to more trafficking of people.
A Home Office spokesperson said: "The policy on an amnesty for illegal immigrants remains unchanged and is very clear - there will be no amnesty, those here illegally should go home.
"We have a proud tradition of offering sanctuary to those who truly need our help, but to grant an amnesty would create a significant pull factor to the UK and would undermine the asylum system as a whole."
According to the National Audit Office, the deportation of all illegal immigrants would cost the UK £4.7bn.
Since 1998, 111,265 illegal entrants have been deported, at the cost to the UK of £11,000 per person.