A Coalition row was raging tonight after Downing Street shrugged off Liberal Democrat criticism of a controversial advertising campaign urging illegal immigrants to go home - and even suggested it could be launched nationwide.
The pilot scheme, which targets London boroughs with large ethnic minority populations, has been disowned by the Lib Dems and denounced by Vince Cable, the Business Secretary, as "stupid and offensive".
But the Prime Minister's spokesman insisted the initiative was "already working" and said it had been approved by the Home Office, which includes a Lib Dem minister, Jeremy Browne.
His claims were strongly denied last night by the party. A spokesman said: "We did not sign it off. No Liberal Democrats were involved."
In recent days large billboards carrying the warning "go home or face arrest" have been driven round six London boroughs on advertising vans. The strongly-worded message has been backed by a leafleting drive.
The Home Office said the scheme was designed to encourage illegal immigrants to avoid the experience of detention and deportation by leaving Britain voluntarily.
Mr Cameron's spokesman said the Prime Minister did not agree with Mr Cable's comments and added: "This pilot that is currently running is about targeting [illegal immigrants] and it is working."
Asked if the pilot could be extended, he replied: "Clearly we will want to look at that in more detail and see how we take this forward."
He insisted the campaign was a Government initiative and had been approved by the "Home Office team". Mr Browne strongly denied the claim and the Lib Dems indicated that it had been conceived by Mark Harper, the Immigration Minister, who is a Conservative.
The Home Office was unable to detail why it believed the scheme was succeeding. A spokesman said: "We have had a great deal of interest in response to the pilot and will evaluate it fully in the longer term."
The Lib Dem peer Lord Oakeshott of Seagrove Bay is to table parliamentary questions asking how the campaign was approved. He said: "This is a totally inappropriate use of public money for party political propaganda."