Only 11 immigrants volunteered to leave Britain after seeing the Government's now-scrapped "go home" vans, an evaluation report has revealed.
The immigration enforcement campaign, which took place between July 22 and August 22 in six London boroughs, saw mobile billboards take to the streets emblazoned with the words "go home or face arrest".
While 11 people left after seeing the vans, t he broader pilot, which also included postcards in shop windows and adverts in newspapers, led to 60 voluntary departures. A further 65 cases are currently being progressed to departure.
In addition, the report reveals the phone number used in the near £10,000 campaign, dubbed Operation Vaken, received a total of 1,561 text messages - but 1,034 were hoaxes.
Around 17 hours of Home Office staff time were required to deal with the hoax messages, the report added.
Last week, Home Secretary Theresa May admitted the vans were "too much of a blunt instrument" and will not be rolled out nationwide.
Shadow home secretary Yvette Cooper said: "The evaluation proves this was never a serious policy to deal with illegal immigration which has been getting worse.
"It was a disgraceful personal error of judgment by Theresa May - she signed off the vans, the slogans and the funding and defended them for months before her recent U-turn."
In addition, some 92 phone calls were received during the operation, of which 13 were hoaxes. Nearly one hour of staff time was required to deal with hoax calls.
In a written statement, Immigration Minister Mark Harper said the 60 departures will save the taxpayer £830,000 - based on the average £15,000 cost of an enforced removal.
The evaluation report also states that each person living illegally in the UK costs roughly £4,250 per year in costs to the public purse - so 60 departures will save a further £255,000.
Andrew Green, chairman of Migration Watch UK, said: "This is the first time that any government has admitted the massive cost to the taxpayer of illegal immigration.
"The annual cost is about six times the amount that the taxpayer spends on the entire immigration system. The case for a firm crackdown on illegals is absolutely undeniable."
Liberal Democrat Business Secretary Vince Cable described the ad-van campaign as ''stupid'', while Labour accused the Government of using language used by the far-right National Front in the 1970s.
The adverts were banned by the Advertising Standards Authority (ASA) earlier this month for using misleading arrest statistics, but cleared over complaints that it was offensive and irresponsible.
The ASA received 224 complaints including some from groups representing migrants in the UK, legal academics and the Labour peer Lord Lipsey.
Mr Harper also revealed there were also no plans to repeat a separate pilot scheme, under which "ask about going home" posters were put up in two immigration reporting centres in Hounslow and Glasgow.
The Immigration Minister added: "The Government will continue to enforce the immigration rules and promote voluntary departure schemes to those who have no right to be in the UK - backed up with arrest, detention and enforced removal where individuals refuse to comply with the immigration rules or present a danger to the UK public."
Downing Street defended the way the scheme had been handled and denied that the Government had shown "poor judgment" over the van campaign.
"I wouldn't accept that at all," the Prime Minister's official spokesman said.
At a regular briefing in Westminster he said: "The right thing to do is to look at a range of ways of tackling illegal immigration. This was one of them. It was piloted and then a decision not to proceed resulted on the basis of the analysis of the pilot."