The UK and EU could face up to six years of gruelling negotiations to broker a comprehensive free trade agreement, Pascal Lamy warned on Thursday (17 March).
The former World Trade Organisation (WTO) director-general also said it will be more costly for the both Britain and the bloc if no deal is reached after the allotted two-year time period.
"Many of us, including me, believe that there is no way removing the egg from the omelette can be done in two years," Lamy told London think-tank Institute for Government.
"So it will take more time, which means we will need some sort of fix. Whether we on the continent call it a continuation clause, whether you on the UK side call it a transition clause."
He added: "Anything that doesn't have an agreement will be more costly. It will be more costly for you and for us because any cost for you has a cost for us." When pressed on how long a free trade deal would take, Lamy said "between five and six years".
The former EU trade commissioner made the comments just a day after EU Council President Donald Tusk warned against a no-deal scenario.
"We will not be intimidated by threats that no Brexit deal is good for UK [and] bad for [the] EU. No deal is bad for everyone, above all for UK," he told his Twitter followers.
"[I] will do everything in my power to make sure that UK, EU are close friends after Brexit and stress that EU's door will always remain open."
UK Prime Minister Theresa May, however, has claimed "'no deal is better than a bad deal for Britain".
Elsewhere, Lamy said there is "room for compromise" between a potential €40bn (£34.7bn) or €60bn divorce bill for the UK, while warning that a separate exit deal for Scotland would add complexity to the already difficult negotiations.
The remarks follow The Queen giving the Article 50 bill Royal Assent. May now has the legal authority to invoke Article 50 of the Lisbon Treaty and trigger Brexit talks. The PM is expected to make the historic move during the final week of March.