A leading trade union boss has made a formal complaint to the Metropolitan Police over a "racist" Brexit poster unveiled by Nigel Farage.

Unison general secretary Dave Prentis has written to the Met about the poster which shows a queue of non-white migrants on the borders of the European Union.

Mr Farage faced almost immediate criticism after revealing the poster in central London, with politicians from across the spectrum condemning the image.

The Ukip leader dismissed suggestions that the poster was racist, but Mr Prentis labelled it a "blatant attempt to incite racial hatred".

He said: "This is scaremongering in its most extreme and vile form.

"Leave campaigners have descended into the gutter with their latest attempt to frighten working people into voting to leave the EU.

"To pretend that migration to the UK is only about people who are not white is to peddle the racism that has no place in a modern, caring society.

"That's why Unison has complained about this blatant attempt to incite racial hatred and breach UK race laws."

Conservative, Labour, Liberal Democrat and Green Party MPs attacked Mr Farage for resorting to "small-minded fear tactics".

Scotland's First Minister Nicola Sturgeon said the poster - which shows people crossing between Croatia and Slovenia to a refugee camp, under a warning that the EU is at "breaking point" - was "disgusting".

Labour's Yvette Cooper said: "Just when you thought Leave campaigners couldn't stoop any lower, they are now exploiting the misery of the Syrian refugee crisis in the most dishonest and immoral way."

Green MP Caroline Lucas said: "Using the innocent victims of a human tragedy for political propaganda is utterly disgusting. Farage is engaging in the politics of the gutter."

Liberal Democrat Tom Brake said: "It's a shame that instead of engaging on the issues, Farage and his cronies have resorted to small-minded fear tactics and xenophobia."

Conservative Neil Carmichael said: "Distasteful propaganda like this can only make our immigration challenges worse, not better, and damage community cohesion in Britain."

Mr Farage launched the poster with a battle bus tour through Westminster, followed by 10 vans plastered with the image.

He suggested that Islamic State (IS, also known as Isis) is exploiting the migrant crisis to flood the continent with terrorists.

"This is a photograph, an accurate, undoctored photograph, taken on October 15 last year following (German chancellor) Angela Merkel's call in the summer and, frankly, if you believe, as I have always believed, that we should open our hearts to genuine refugees, that's one thing," Mr Farage said.

"But, frankly, as you can see from this picture, most of the people coming are young males and, yes, they may be coming from countries that are not in a very happy state, they may be coming from places that are poorer than us, but the EU has made a fundamental error that risks the security of everybody."

Told that the people were refugees, he said: "You don't know that - they are coming from all over the world.

"If you get back to the Geneva Convention definition, you will find very few people that came into Europe last year would actually qualify as genuine refugees."

He added: "When Isis say they will use the migrant crisis to flood the continent with their jihadi terrorists they probably mean it."