The use of vans with adverts urging illegal immigrants to "go home or face arrest" are working, No 10 says.
The PM's official spokesman said David Cameron disagreed with Lib Dem Business Secretary Vince Cable's view that the scheme was "stupid and offensive".
The spokesman said the Home Office was "clear that this is already working".
He did not give figures on levels of response but said the Home Office was "looking at what they can take forward" from the London pilot scheme.
The Home Office say the van pilot had only ended on Sunday so were not sure "what figures have been collated at this point".
The pilot scheme saw vans driven through Barnet, Hounslow, Barking and Dagenham, Ealing, Brent and Redbridge featuring the "go home" posters - which had a text number illegal immigrants were encouraged to use to arrange their departure.
Leaflets, posters and messages in local newspapers were also used carrying the same message.
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.
The use of the vans have been widely criticised - Brent's council leader Muhammed Butt said the plan 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".
Mr Cable told the BBC's Andrew Marr Show on Sunday: "It was stupid and offensive. I think it is very unlikely it will continue."
He questioned whether illegal immigrants would have enough of a "sophisticated grasp of English" to be able to read the posters on the vans at a distance.
"It is designed, apparently, to sort of create a sense of fear (in the) British population that we have a vast problem with illegal immigration," he said.
"We have a problem but it's not a vast one. It's got to be dealt with in a measured way dealing with the underlying causes."
The Home Office said the scheme was an alternative to criminal procedures and the areas were chosen because they had either significantly higher or below average numbers of voluntary returns.
It said voluntary returns were the most cost-effective way of removing illegal immigrants and saving the taxpayer money.
Councils were not consulted as it is an immigration policy which does not require local consent, it added.
The Downing Street spokesman said the government would wait until final information on the pilots was available before deciding whether to extend the scheme nationwide.
Public and Commercial Services union general secretary Mark Serwotka said: "It is simply not credible that the government has enough information to state that these billboards are 'working' - in fact, it is far from clear how any success could even be measured."