Liam Byrne, the Immigration Minister, disclosed the move last night and did not rule out allowing illegal immigrants to legitimise their position in the country.

Mr Byrne told MPs that he had asked officials to prepare a report on the issues surrounding an amnesty for illegal immigrants. When asked whether he was ruling out an amnesty, he told MPs on the Commons Home Affairs Select Committee: “It is too early to tell.” He added that he did not wish to mislead the committee by giving an answer that later proved incorrect.

Mr Byrne was twice asked about the possibility of an amnesty and on neither occasion did he rule it out, as previous home secretaries and immigration ministers have done.

Mr Byrne told MPs investigating immigration controls: “The position I’m in is really needing to understand in more detail the precise segmentation of people whose positions have not been regularised. I have commissioned some analysis. I need to understand issues like the length of stay already here, how many people are here for X, Y and Z. I need to understand where they came from, what process their cases are in.”

David Davis, the Shadow Home Secretary, said that this might encourage migrants to head for Britain. He said: “With our borders so badly protected, even speculating about an amnesty is highly irresponsible. An amnesty could lead to a massive and uncontrolled increase in the numbers coming here.”

Richard Benyon, a Conservative member of the select committee, expressed concern about Mr Byrne’s apparent change of tack on an amnesty. “There was a clear change of emphasis,” Mr Benyon said.

“There was a change in language and it certainly rings alarm bells with me that it is not being ruled out in the very least and is being actively considered at the very most. It will not solve the problem, it will make it worse.”

The number of illegal immigrants in Britain is officially estimated at between 310,000 and 570,000, though last month Dave Roberts, the head of enforcement at the Immigration Service, provoked a storm of criticism when he admitted that he did not have the “faintest idea” how many illegal migrants were in the country.

Jack Dromey, the deputy general-secretary of the Transport and General Workers Union, said last month that it would be “impracticable and immoral” to try to deport half a million illegal immigrants, and backed an amnesty.

Three years ago David Blunkett, the former Home Secretary, suggested that the introduction of identity cards could be a way of encouraging people to regularise their position in the country. “I am not offering a moratorium so that everyone from everywhere arrives and does it. I want to deal sensibly with those who are prepared to put their hand up,” Mr Blunkett said.

Although the idea of an amnesty will provoke controversy, Labour gave an amnesty in 2003 to 15,000 families — about 50,000 people in all — who had sought asylum before 2000. It was the third asylum amnesty since 1991. Together, they have resulted in at least 103,000 people being allowed to stay.

Sir Andrew Green, the chairman of the Migrationwatch think-tank, said that Mr Byrne’s comment were irresponsible. He said: “It is not a change of thinking, it is an absence of thinking. Amnesties are an absurd idea. The Italians have given five amnesties and the Spanish six, yet the number of applications increased on every occasion. Amnesties do not reduce illegal immigration, they encourage it.”

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