Renegotiating a trade deal with the European Union could take the UK up to a decade to finalise after its "potentially catastrophic" decision to leave the bloc, a trade expert has warned.

Jason Langrish, executive director of the Canada Europe Roundtable for Business, said the lack of a clear exit strategy at Whitehall is a cause for concern three months before formal Brexit talks get underway.

He accused UK negotiators of remaining in "campaign mode" and said that they were unlikely to minimise the economic pain arising from Brexit.

The remarks come days after Sir Ivan Rogers quit as the UK's ambassador and criticised Whitehall for "muddled thinking" in a memo to staff.

"I have not formally advised the UK government but I have had a number of informal discussions with officials," Langrish, who worked on Canada's free trade deal with the EU, wrote in the Observer.

"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 very 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."

Langrish added that without flexibility from Whitehall on freedom of movement and the role of the European courts, a post-Brexit trade deal would probably adhere to World Trade Organisation rules, triggering a 4% drop in UK GDP versus a 1% loss in the EU.

He pointed to the trade deal signed between the EU and Canada last year, which took almost a decade to negotiate, and suggested that a two-year time frame for a Brexit trade deal was unrealistic.

"There is no mutually beneficial deal available between the UK and the EU in this time frame," Langrish wrote.

"Existing levels of trade and investment will not guarantee a positive outcome for the UK. The Brexiters who think they have the upper hand are wrong."