British politicians have "made a terrible mistake" in failing to take on the argument about immigration, David Cameron's chief confidant has said.

Discussing the incendiary language used to discuss immigration and citing Ukip's "Breaking Point" poster as an example, Sir Oliver Letwin thinks mainstream politicians are to blame for the tone of the debate.

"We all, the Labour party and the Conservative Party alike ... made a terrible mistake, which was not to take on the argument about migration," he told The Times.

"We just accepted the line taken by some ... that somehow migration was in itself a threat. On the contrary, properly controlled migration enriches the country in every sense."

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The former head of the Government's Brexit preparations said Prime Minister Theresa May must now do more to change the terms of discussion and challenge the xenophobic mood.

“I would like politicians on all sides to do that and I would like that to include the prime minister,” he added.

“What’s important is that ... we are talking about [immigration] in a balanced way and bringing in the advantages as well as recognising the pressures.”

He said Ukip exploited the failure of mainstream politicians to "put the counter-argument".

Farage refuses to apologise for 'Breaking Point' poster

Mr Letwin has urged Ms May to drop the Government's appeal against a court ruling that parliament must approve the process to trigger Brexit.

He told BBC Radio 4 the Government should scrap the appeal and instead deliver a "fast and tightly timetabled and constrained bill" to parliament, which would allow the government to start the exit process quickly.