Boris Johnson wants to consider an illegal immigrant amnesty
The Mayor of London is to commission a study of a potential amnesty for illegal immigrants in the capital.
Boris Johnson said deporting thousands of people working illegally in the UK was "just not going to happen".
He told Channel 4 News that allowing long-term illegal immigrants to earn the right to stay would see "hugely increased" tax revenues.
Immigration Minister Phil Woolas said the suggestion was "naive" and would lead to more trafficking of people.
Mr Woolas said: "I think this is naive of the Mayor. His comments might start with the best of intentions but will lead to more people traffickers making more money and exploiting more vulnerable individuals."
Of the 700,000 illegal immigrants thought to be in the UK, about 400,000 are in London.
Mr Johnson said: "What I want to do is to commission a study by my own economics team here at the Greater London Authority into the possibility [of an amnesty]."
"We want to look in detail at what the economic impact of such an earned amnesty system would be."
He acknowledged that illegal immigrants had broken the law and should "in principle" be deported, but he added: "Unfortunately it is just not going to happen."
Mr Johnson said he did not want to encourage illegal immigration but said there were significant legal and financial obstacles to mass deportations.
He suggested that those allowed to stay would have to have at least five years' residency. They must also be able to demonstrate their commitment "to this society and to this economy".
He added: "There's got to be a very substantial period in which they have been in this country.
"I think that we could have other hoops that they might have to go through in order to be able to qualify for an earned amnesty scheme.
'Every eight minutes'
"For instance, it might be necessary to have a clean criminal record. It might be important that they should go through various citizenship tests, the kind we already have.
"And there might be some sort of financial obligations that they have to meet as well."
Migrationwatch UK said the proposal was "amazing" with a recession looming.
Chairman Sir Andrew Green said: "An amnesty would also add hundreds of thousands to the housing lists."
Mr Woolas said the UK Border Agency was committed to stopping illegal migration.
"We are putting in place the biggest shake-up of the immigration system for 45 years and we are seeing the results of this."
He added: "We are putting more resources into expelling foreign law breakers and last year we removed one person every eight minutes."