NICK Clegg will today attack David Cameron's "fixation" with cutting net migration to tens of thousands, suggesting it is hindering efforts on illegal immigration.

The intervention by the Deputy Prime Minister and Liberal Democrat leader comes just weeks after his party's Business Secretary Vince Cable triggered a cabinet row by saying the target was damaging the economy.

Mr Clegg is also expected to use a speech later today to say migrants from new EU countries should have to wait more than seven years before they can work in the UK.

He will also promise a clampdown on sham marriages.

Labour last night accused Mr Clegg of failing to act on immigration controls during his four years in office.

In his speech, Mr Clegg will say the Tory obsession with the target stopped ministers focussing on a more important issue - effective border controls.

He will accuse his Tory colleagues of having quietly backed away from pledges on exit checks, as he promises to put them in his party's manifesto for next year's general election.

Mr Clegg will say: "We can marry our ideals about the open, welcoming Britain we love with the realities of running a effective immigration system.

"We can keep Britain open-hearted, open-minded and open-for-business and at the same time ensure our hospitality isn't open to abuse. It just takes a lot of work."

He will also claim the Conservatives have now "softened their attachment" to the net migration target, pointing out it was missing from an article the Prime Minister wrote on immigration last week. But he will say more needs to be done to eradicate illegal immigration.

Part of the problem is people arriving legitimately but then overstaying their visa, he will warn. But without exit checks it is impossible to tell who has left and who has stayed.

He will also claim that before the 2010 election around 57 per cent of entry and exit points were covered by such checks, a figure which has risen to 80 per cent.