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Boris Johnson today backed the controversial “Go home or face arrest” poster campaign that has been branded intimidating and offensive by Nick Clegg’s Liberal Democrats.

London’s mayor insisted it was “not racist” to tell illegal immigrants to leave the country or to say that they were breaking the law by being here.

His comments stoked a blazing Coalition row over the campaign that is being piloted in six London boroughs, involving vans advertising a helpline number to migrants if they leave the UK voluntarily.

Mr Johnson wrote in his weekly newspaper column that the posters were “blunt” and “uncompromising”. However, responding to people who had branded them racist, he said: “On that point I am afraid I have to disagree. Illegal immigrants have every opportunity to make their case to remain in Britain.”

He said a tiny number were expelled, which meant a virtual amnesty operated for the rest. “It is certainly not racist to point out this absurdity,” he added.

A source close to Mr Clegg responded: “He is wrong. This is something we do not agree on and we do not think should have happened in this way.”

The posters have been widely condemned. Ukip Nigel Farage branded them “nasty Big Brother” ads, claiming they were an attempt by the Tories to stop activists defecting. Vince Cable called them “stupid and offensive”.

Liberal Democrat sources today revealed that the campaign did not go for approval to the Cabinet’s home affairs committee, which Mr Clegg chairs, on the grounds it was not a decision about policy but on how to implement policy. “It is a comms matter,” said the source.

The distinction will fuel speculation that the posters may have been influenced by Tory strategist Lynton Crosby, who has put immigration concerns at the top of the Conservative election fightback.

Mr Clegg, who has returned to work today after a holiday, has complained about the ads to the Home Office but is waiting to see how the pilot scheme goes before saying if he will try to block an expansion of the scheme.

Shadow home secretary Yvette Cooper said: “There is now complete confusion over whether these ill-judged ad vans are actually government policy or not. This is a cynical stunt from a Government that’s not getting the basics right on immigration.”

The ad vans are targeting several London boroughs where Ukip has done well, including Hounslow, where a group of Conservative councillors recently defected. Other boroughs targeted are Barking & Dagenham, Ealing, Barnet, Brent and Redbridge.