Nick Clegg, the deputy prime minister, has laid into a Home Office scheme that saw vans with a poster telling illegal immigrants to "go home" drive around less wealthy parts of London.
The campaign, currently running in six London boroughs, features leaflets and posters that say: "In the UK illegally? Go home or face arrest." The vans had been driving around displaying a large version of the poster, but the Home Office said the pilot had now ended.
Clegg insisted no Lib Dems - the junior partner in the coalition - had been made aware of the plan beforehand.
On a BBC Radio 5 Live phone-in 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 ... 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 reinstating as part of our coalition agreement."
The Lib Dem business secretary Vince Cable had described the van-based campaign as "stupid and offensive" but was slapped down by No 10, with the prime minister's spokesman claiming it was already working.
The Home Office said the impact of the vans, which finished driving around London on Sunday, was yet to be fully assessed as a poster and leaflet campaign was due to continue for another three weeks.
The campaign has been widely criticised outside the coalition, with even Nigel Farage, leader of the anti-immigration Ukip party describing it as "nasty, unpleasant ... I don't think messages like this will make any difference."
The shadow home office minister, Baroness Smith said: "As we have repeatedly said, these ill-judged ad vans are cynical and stupid politics from a Government not getting the basics right on immigration.
"Nick Clegg has now admitted the Home Office have failed to make much progress on the basics, yet he has added to the chaos and confusion within Government over the ill-judged ads.
"If he opposes these ad vans and has spoken to the immigration minister, will they continue? And did his own Lib Dem minister in the Home Office sign them off?
"This is just the latest desperate attempt by Nick Clegg to distract from his record propping up this Tory-led government. It won't work because he and David Cameron are joined at the hip."