Boris Johnson's advisers are to study the potential benefits of an amnesty for illegal immigrants, the Mayor of London has revealed.
He moved to reopen the debate over how to deal with hundreds of thousands of people working illegally in the UK, insisting that simply deporting them was "just not going to happen".
Mr Johnson's comments risk opening a rift between himself and Tory leader David Cameron, with whom he openly clashed on the issue of an amnesty earlier this year.
Speaking to Channel 4 News, the Conservative mayor said that allowing long-term illegal immigrants to earn the right to stay in Britain would see "hugely increased" tax revenues.
Of the 700,000 thought to be working illegally in the UK, about 400,000 are in London.
"What I want to do is to commission a study by my own economics team here at the Greater London Authority into the possibility," he said.
"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 insisted that he did not want to incentivise illegal immigration but said there were significant legal and financial obstacles to mass deportations.
He suggested that those allowed to stay would have to be at least five years resident and able to demonstrate their commitment "to this society and to this economy."
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