The Liberal Democrats were last night branded irresponsible by the Conservatives for proposing a policy of "earned legalisation" to stay in Britain for as many as 600,000 illegal immigrants.

Nick Clegg, the LibDems' home affairs spokesman, flagged up the initiative ahead of a debate on the issue of immigration at the party's annual conference in Brighton next month.

In a newspaper article, he called for Britain to have stronger border control. However, it was his reference to "earned legalisation" for illegal immigrants, which sparked a broadside from the Tories.

Mr Clegg wrote that neither Labour nor the Conservatives had anything to say about the large number of "irregular residents who live in a twilight world of illegality and exploitation". He asked if Labour and the Tories seriously proposed deporting all 600,000 individuals, implying such a policy would be impracticable.

"A route of earned legalisation should be made available to those who have lived here unauthorised for many years," argued Mr Clegg. "We should set stringent criteria - this is not a blanket amnesty - namely that the applicant should have lived in the UK for many years, should have a clean criminal record and should show a long-term commitment to the UK."

However, David Davis for the Tories responded by saying: "This is irresponsible because on the one hand it will encourage people to come here illegally as well as being unfair to those who have obeyed the law and tried to enter the UK legally."