Media playback is unsupported on your device Media caption Deputy PM Nick Clegg: "The bonds would need to be well targeted - so that they don't unfairly discriminate"

Nick Clegg has made his biggest intervention to date in the debate on immigration, calling for cash deposits of more than £1,000 for some migrants.

The deposit would be paid by visa applicants from "high risk" countries and repaid when they leave the UK.

The deputy prime minister said migrants made a huge contribution but there must be "zero tolerance" of abuses.

"Mainstream" parties had to "wrestle the issue from populists and extremists," he added.

It came as Mr Clegg's Liberal Democrat colleague, Vince Cable, disowned a target of reducing net migration to below 100,000 by 2015, saying it was a Conservative policy, not a coalition one.

He also said the immigration visa system should be "easy" and "flexible".

'Discriminatory'

Mr Clegg insisted the business secretary was fully behind the security bond idea and that they also agreed on the issue of the migration cap, despite Mr Cable's stronger criticism of Tory coalition colleagues.

Analysis The Lib Dems wanted to show they too could be tough. So they threw out a much-attacked party policy - giving illegal immigrants citizenship. They backed a robust-sounding government plan - deposits for migrants. It was bad luck then that Vince Cable comments about the "enormous damage" that could be caused by low net migration emerged the night before. We're reassured that Vince agrees with Nick. But that is not the point. The worry for the Lib Dems is that when immigration is mentioned the party is only ever heard worrying that rules might be too strict. Nick Clegg's speech was meant to change that. Vince Cable's words have muddied his message.

The security bond idea - designed to tackle the problem of people coming to the UK for holidays or as students and remaining in the country illegally - was floated several times by the previous Labour government but never implemented and was understood to be under development by the Home Office.

It has been criticised in the past as "half-baked" and "clearly discriminatory" by Lib Dem deputy leader Simon Hughes but Mr Clegg urged critics to reconsider it, saying it could be a "useful, additional tool".

"If we get this right, there is no reason why this cannot make the system work more efficiently," he said.

Labour had a "lamentable record" on immigration, he added, and "just because they could not get it right does not mean we cannot do it better".

'Retaliation'

But former Labour minister Keith Vaz, now chairman of the Home Affairs Select Committee, said the idea was "unworkable, impractical and also discriminatory" and said it had gone down badly when he raised it in India in the late 1990s.

"This idea is likely to end in tears...we have tried it before" he told the BBC's Daily Politics.

The bond, he added, would not deter some people from trying to stay on after their visas ended, or address the cost of having to remove them, while it would have repercussions for UK relations with other countries.

"We have to choose the countries we want to target," he told the BBC's Daily Politics. "They are going to be very angry. They are likely to retaliate against Britain."

Media playback is unsupported on your device Media caption Keith Vaz: We must avoid 'immigration arms race'

Mr Vaz urged all the party leaders to avoid a rhetorical "arms race" on immigration.

Labour leader Ed Miliband has reshaped his party's policy in recent months and David Cameron will set out what the government has done to tighten controls on Monday.

In his speech, Mr Clegg pledged to "lay the foundations for an immigration system that embodies this nation's instincts and its values" of tolerance and openness but one that also commanded public support.

'No contradiction'

The deputy prime minister also revealed plans to increase cash penalties for "unscrupulous" employers who knowingly hire illegal immigrants because they are cheaper. The maximum fine is £10,000 per illegal worker and Mr Clegg called for a doubling of penalties.

Shadow immigration minister Chris Bryant said politicians should "tackle abuse and not just talk about it", saying his party's proposals for ending student visa loopholes and penalising firms who hire illegal workers were "practical and workable".

Media playback is unsupported on your device Media caption Labour's Immigration Minister Chris Bryant: Government "focussing on the wrong end of problem"

Mr Clegg's intervention came as Vince Cable lashed out at Home Secretary Theresa May's target of cutting net migration to below 100,000 by 2015 saying it was a Tory and not a Lib Dem policy and could have disastrous consequences for the economy.

He also suggested, in an interview with The House magazine, that Tory ministers were being disingenuous to quote progress towards the target as an example of the government getting to grips with immigration.

"We have obviously no control over the European Union and that is actually where much of the movement comes. And a lot of the public anxiety which is experienced in by-elections and elsewhere has actually been about people from Eastern Europe.

"Now, you can argue whether that's a good thing or a bad thing but it's got nothing to do with the non-EU, which is the area which is controlled by government.

"The reducing to under 100,000 is not government policy and it would be unattainable without, if it was attainable enormous damage would be done, notably through overseas students, which is one of the biggest components, actually."

Mr Cable denied putting out a contradictory message to Mr Clegg, who had hailed the coalition's success in cutting net migration "by a third" in his speech.

"Nick and I talk often and we are on the same page. I'm simply emphasising that Britain has got be open for business or we can't succeed as an economy," the business secretary told BBC News.