EU referendum: Iain Duncan Smith claims Number 10 is 'bullying' Britain to stay in Europe

The UK's EU Remain campaign is conducting itself in a "troubling" manner and risks damaging the government after the referendum in June, the pro-exit work and pensions secretary Iain Duncan Smith has said.

He has described how the integrity was at stake of those who made "unsubstantiated" claims about what would happen to the UK were it to leave the EU, saying that the In camp was basing its campaign on "spin, smears and threats".

Writing in The Daily Mail, Duncan Smith, a former Conservative Party leader said: "The acrimonious manner in which all this has been conducted is troubling, and will I fear have consequences long beyond 23 June."

Duncan Smith criticised the series of "highly questionable dossiers threatening almost biblical consequences if we dare to consider a future outside the European Union.

"After all, such desperate and unsubstantiated claims are now being made that they begin to damage the very integrity of those who make them in the eyes of the public."

His comments will be seen as criticism of David Cameron and his allies and comes in a week when, as ammunition for their case, the In Campaign outlined how Britain's trade, economy and security would be at risk in the event of a Brexit.

It also comes a day after an alarming warning was sounded by French president Francois Hollande who after meeting David Cameron in Amiens, said there would be "consequences" for immigration and the economy if it left.

French economy minister Emmanuel Macron had earlier suggested the Calais refugee camps could come to Britain as the Le Touquet treaty, where the UK has border police in France, could be torn up.

This possibility was dismissed by London mayor Boris Johnson who said: "I would say: Donnez-moi un break. There's absolutely no reason why that treaty should be changed."

"It was an intergovernmental treaty; it was the Le Touquet treaty. It was signed between the British government and the French government. It's not in the French interests to want to do that and it's just the usual flapping and scaremongering," he said, according to The Guardian.