Deputy Prime Minister Nick Clegg has criticised a Home Office scheme which saw vans bearing billboards encouraging illegal immigrants to return home and insisted no Liberal Democrats were aware of the plan.

Mr Clegg challenged Tory Immigration Minister Mark Harper over the pilot scheme, which saw the vehicles travelling around parts of London, and urged him to focus on introducing proper entry and exit checks at the borders.

The Lib Dem leader said that he did not think the scheme was a "clever way of dealing with this issue" just a day after Downing Street defended the controversial initiative.

On a BBC Radio 5 Live phone-in Mr Clegg said he was "very surprised" by the pilot scheme and added: "I think people should play by the rules and play by the law. I want to see us make sure that the public have confidence in the immigration system, that it works properly.

"I don't happen to think that having a couple of vans driving around North London is the way of actually inspiring public confidence that we have an immigration system that is working properly.

"What I want to see the Home Office concentrate on instead is making sure that we have a simple system, that's done in many other countries, where you count people in and you count people out."

He added: "I said to the Home Office in very clear terms, I was talking to the Immigration Minister yesterday - he's a very good guy, by the way, Mark Harper - I was saying the effort that should be spent is not on vans drifting around North London but on actually reinstating exit checks which I think were wrongly removed in the past and which we are committed to, and this was on my personal insistence, committed to reinstating as part of our coalition agreement."

Lib Dem Business Secretary Vince Cable described the van-based campaign as "stupid and offensive" but was slapped down by No 10 over his comments, with the Prime Minister's spokesman claiming it was already "working".