Number 10 has insisted the government has "no plans" to launch an amnesty for illegal immigrants.

Former home secretary David Blunkett suggested on Wednesday that such an amnesty could be introduced once identity cards are in place.

His comments came after immigration minister Liam Byrne refused to rule out the idea that has been advocated by think tanks and unions.

But the prime minister's official spokesman said: "There are no plans for an amnesty."

Asked why the minister had not ruled the move out, he added: "Liam Byrne was doing the proper job of a minister in making sure he has all the facts in front of him."

Blunkett said he had considered introducing the policy when in office in 2003, as a means of bringing the approximately 500,000 'underground' workers into the open.

But he put a decision off until ID cards, the plans for which have now received royal assent, were ready.

"You firstly don't have any incentives for people to come up front and register and make themselves available," Blunkett told the BBC Radio 4 Today programme.

"And secondly, you have no means of tracking them. And what you can't do is announce an amnesty in advance of the date of the amnesty, because obviously you then act as a magnet for pulling people into the country."

The Labour MP conceded that Byrne had signalled that an amnesty may be on the cards by failing to dismiss the suggestion.

However, he added that the government would have been criticised if the minister had ruled it out before introducing the policy.

Byrne told the Commons home affairs select committee on Tuesday that it was "too early to tell" whether he would give the go-ahead to an amnesty.

He revealed that he had asked officials to prepare a report on the "issues" around allowing illegal immigrants to register their presence in the country, to pay tax and to receive services.

"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.

"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."

The idea was welcomed by Transport and General Workers' Union deputy general secretary Jack Dromey, who argued that it would be "impractical and immoral" to try to deport every illegal immigrant.

"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," he said.

"They are good men and women, essential to our economy, and pillars of their local churches and communities."