LONDON — Britain's advertising watchdog said Friday it had launched a formal investigation into a government campaign urging illegal immigrants to "go home or face arrest," after receiving dozens of complaints that it was racist.

Trucks carrying billboards with the words, "In the UK illegally? Go home or face arrest" drove through London for a week last month in a pilot scheme by the interior ministry that has sparked widespread criticism.

In a campaign that critics have dubbed the "racist van", the trucks were driven through several areas of London with large ethnic minority populations.

Posters, leaflets and advertisements in local newspapers are still carrying the message as part of the campaign.

"The Advertising Standards Authority has launched a formal investigation into the Home Office 'Go Home' ad campaign following 60 complaints," a spokesman for the watchdog said.

Complainants said the phrase "go home" was particularly "offensive and irresponsible" as it was reminiscent of slogans used by British racist groups in the past, he added.

The adverts feature a huge picture of a pair of handcuffs together with a number said to represent the number of arrests in the local area. They tell people to text the word "HOME" to a given phone number to get help to leave Britain.

A spokesman for the Home Office, the interior ministry, said it was in contact with the ASA over the investigation and would respond "in due course."

Prime Minister David Cameron's spokesman defended the vans last week, saying it was "clear that this is already working" and that helping illegal immigrants to leave voluntarily was the most cost-effective way to reduce their numbers.

But the campaign has caused a rift in Cameron's Conservative-led coalition government, with several senior members of junior partner the Liberal Democrats, blasting it as offensive.

For years, immigration, along with the economy, has regularly topped polls of voters' concerns.

It remains a key issue with less than two years until Britain's next general election, as the center-right Conservatives fear a growing threat from the UK Independence Party, which campaigns for migration curbs and an exit from the European Union.

A Home Office spokesman said the ministry could not provide a figure for the number of illegal immigrants living in Britain, but in 2005 it estimated that it could be up to 570,000.

Some 14,000 migrants were forcibly removed from Britain in the year ended March 2013, while 28,000 left voluntarily, according to ministry figures.