Deputy Prime Minister Nick Clegg criticises a Home Office scheme in which vans bore billboards encouraging illegal immigrants to return "home", insisting no Liberal Democrats were aware of the plan.
The adverts on the vans said: "In the UK illegally? Go home or face arrest. Text HOME to 78070 for free advice, and help with travel documents. We can help you return home voluntarily without fear of arrest or detention."
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.
The pilot scheme cost just under £10,000, which is cheaper than forcibly removing someone. That process costs about £15,000, according to the Home Office.
On a BBC Radio 5 Live phone-in on Tuesday 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.
'You count people in and you count people out'
"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."
The poster vans were driven around Hounslow, Barking and Dagenham, Ealing, Barnet, Brent and Redbridge.
The areas were chosen because they have either significantly higher or below average numbers of voluntary returns, meaning that the success of the pilot can be assessed, claimed the Home Office.
The use of the vans has been widely criticised - Brent's council leader Muhammed Butt said it was an "act of desperation" and that more needed to be done to process people's claims.
He added it would "just drive people underground".
A report published last weekend said official UK migration statistics are "not fit for purpose" and "little better than a best guess."
It follows controversial comments made by Conservative Business Minister Matthew Hancock last week in which he said UK companies have a "social duty" to hire young British workers rather than better-qualified immigrants.
Mr Clegg added it was "frustrating" that the Home Office had not acted on implementing exit checks.
'Make sure there aren't loopholes'
"It is frustrating to me that the Home Office has not yet made as much progress on this as they should do under the terms of our coalition agreement," he said.
"The way to bolster public confidence in an immigration system which allows people to come into this country who want to make a contribution, who want to pay their taxes, play by the rules, but make sure there isn't abuse, there aren't loopholes, there isn't illegal behaviour, is to ensure that we reintroduce exit checks.
"That hasn't happened yet and my view is that is where that attention of the Home Office should be devoted."
A Home Office spokesman said the impact of the vans, which finished their advertising patrols on Sunday, was yet to be fully assessed as a poster and leaflet campaign was due to continue for another three weeks.