The prospect of an amnesty for illegal immigrants has been raised by the new minister in charge of immigration.
Could an amnesty replace efforts to remove illegal migrants?
Liam Byrne told MPs he had asked for a report on the issues around a possible amnesty, saying it was "too early" to rule out the controversial idea.
There is no official estimate of illegal immigrant numbers. Unofficial estimates vary from 310,000 to 870,000.
Shadow home secretary David Davis said speculating about an amnesty was "highly irresponsible".
Downing Street said there were no plans for an amnesty, saying Mr Byrne was "doing the proper job of a minister which was to make sure he has all the facts and analysis appropriate".
In an appearance before the Commons home affairs select committee, Mr Byrne was asked whether the UK should follow countries like Spain in holding an amnesty.
He told the MPs: "The position I'm in is really needing to understand in more detail than I do at the moment the precise segmentation of people whose positions have not been regularised."
He said he would wait for a report before making any decisions on an amnesty.
Liam Byrne took over 11 days ago
"We do, as an agency (the Immigration and Nationality Directorate) need to bring forward a stronger enforcement and removal strategy, that is actually rooted in an analysis of the kind of problems that it's trying to solve.
"And so, to be frank with the committee, it's too early for me to get into that question yet, because I just haven't got that analysis in front of me."
The Institute for Public Policy Research, a think tank close to ministers, has said an amnesty to 500,000 people living illegally in the UK could raise £1bn in taxes.
And Jack Dromey, deputy general secretary of the Transport and General Workers Union, recently said it was "impractical and immoral" to deport 500,000 illegal migrants.
Mr Dromey said: "The time has come for the government to be brave, distinguishing between deporting the few who commit serious crimes and allowing the many undocumented migrant workers to remain.
"They are good men and women, essential to our economy and pillars of their local churches and communities."
Mr Byrne's predecessor as immigration minister, Tony McNulty, rejected the amnesty idea earlier this year, saying he would simply not accept illegal immigration.
The Conservative spokesman David Davis said even speculating about a possible amnesty was "highly irresponsible" when the UK's borders were so badly protected.
"An amnesty could lead to a massive and uncontrolled increase in the numbers coming here," said Mr Davis.
Pressure group Migrationwatch said it was "an absurd idea".
Its chairman, Sir Andrew Green, said: "The clear evidence is that amnesties make a bad situation worse. They are also extremely expensive for the tax payer.
"For a start, an amnesty would add half a million people to the housing lists as the local authorities would become responsible for their housing."
Former Home Secretary David Blunkett said: "It's impossible to have an amnesty without ID cards and a clean database."
Mr Blunkett said ministers would make the UK a "magnet" for illegal migrants if they announced an amnesty before it actually came into force.