Britain risks a “catastrophic” Brexit because the government is so dismissive of the concerns of trade experts, according to one of the figures behind the EU-Canada trade deal which took a decade to negotiate.

Writing in the Observer, Jason Langrish – one of Canada’s authorities in the field – says the UK’s former ambassador to the EU, Sir Ivan Rogers, who resigned last week and quit the civil service, was absolutely right to say that a British deal could also take a decade to strike.

Langrish, who was closely involved in the prolonged Canadian talks, argues that Rogers’ analysis of the time-scale “seems realistic”, and says discussions he has had with UK government officials about Brexit suggest that there is little chance of minimising serious potential damage from the UK’s exit from the European Union.

Although he has no formal role in advising the UK, Langrish has been sounded out behind the scenes by those involved with handling Brexit in Whitehall. The impression he has been left with is that unless the British government shows more flexibility it will probably have to revert to World Trade Organisation rules and common tariffs, which could lop 4% off UK GDP.

Referring to his talks with UK officials, he writes: “While they have always been pleasant (and notably friendly towards Canada), my view is that they remain in campaign mode.

“Were they willing to realistically discuss options for Brexit, as opposed to telling you what they intend to do in a general sense while dismissing the obvious concerns, they may have a chance to minimise the damage from the potentially catastrophic decision to leave. This seems increasingly unlikely. Let’s hope that the courts, parliament and, ultimately, the electorate do it for them.”

The Comprehensive Economic and Trade Agreement has been cited by UK politicians and senior figures in other member states as a potential model for the UK to follow. In March, before the Brexit referendum, Boris Johnson, now the foreign secretary, said: “I think we can strike a deal as the Canadians have done, based on trade and getting rid of tariffs. It’s a very, very bright future I see.”

Rogers was subject to a barrage of criticism from hardline Brexiters when it emerged that he had warned ministers that the view in Brussels was that a trade deal with the EU could be a decade at least in the making, and that even then there was no guarantee of success.

Rogers quit his post last week and in a memo to staff talked of “muddled thinking” about strategy at the top of government.

A spokesman for the Foreign Office said that Rogers had also resigned from the civil service and would be paid three months’ salary in lieu of his notice period. He will not receive a payoff and did not ask for one.

“We are grateful for Sir Ivan’s work in Brussels and across a number of other senior positions in the civil service,” the spokesman said.