An amnesty to allow illegal immigrants to seek British citizenship was backed by the Liberal Democrats yesterday as Nick Clegg, the party’s home affairs spokesman, insisted that immigration was good for Britain.

Mr Clegg attacked David Cameron, who criticised levels of immigration in Britain as too high last month, and told the Lib Dem conference that immigration was an opportunity, not a threat. “In our view it is not too high,” he told the BBC. “In our view the whole debate on immigration is placed on the wrong footing if you try to delude yourself or the public into thinking there is a kind of ideal number.

“Our view is that if you have an immigration system where you have large numbers of people coming into this country, the system is not competent, and you don’t plan for the consequences, and you don’t work to integrate people, then of course the numbers can seem too high. You have to make immigration work for you.”

The conference backed a proposal to offer “irregular migrants” who have been in Britain for ten years to apply for citizenship, initially by being given a two-year work permit, provided a series of conditions were met. These included having no criminal convictions; payment of a fee or completion of community service or voluntary work; and passing tests in speaking English – or, at the insistence of the conference, Welsh.

Mr Clegg said that there were about 570,000 people working in the black economy without paying tax and the cost of deporting each would be £11,000.

The policy was immediately attacked by Labour and the Conservatives, who argued that such an amnesty would attack legal migrants to Britain. Liam Byrne, the Immigration Minister, said: “I believe those here illegally should go home, not go to the front of the queue for jobs and benefits. That is why we are now deporting someone every eight minutes and doubling our frontline enforcement resources.”

Damian Green, the Conservatives’ immigration spokesman, said: “The Lib Dems are living in a fantasy world. These proposals will encourage people to break the law and enter the UK illegally. Other countries have tried amnesties and have had to have five or more. This will send out a message that Britain’s borders are well and truly open to everyone in the world.”

Lib Dem delegates rejected an attempt to dilute the terms of their proposed amnesty. Lord Roberts of Llandudno told the conference that ten years to qualify for British citizenship was too long because such people were among the most vulnerable in society and they and their children needed healthcare, benefits and protection. A motion to scrap the ten-year time period was defeated.

In other areas the Lib Dems backed tougher immigration rules, proposing the reintroduction of exit checks at ports and airports that were scrapped in 1997, a border force and a fee system for work permits whereby employers recruiting higher-paid workers from outside the European Union would pay more.

A separate attempt to scrap the call for exit checks was also defeated by the conference.

Lord Teverson, a former Liberal Democrat MEP, said that such a policy smacked of “fortress Britain” and that Britain should join Europe’s border controls. “Fortress Britain and fortress Europe are not part of the 21st century,” he told the conference. An amendment to drop the plan was easily defeated.

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