A comprehensive post-Brexit trade deal could take up to six years to negotiate, the former head of the World Trade Organisation (WTO) has told ITV News.
Pascal Lamy also warned that failing to agree any deal before Britain's EU exit will only hurt consumers.
The UK government, which is due to trigger Article 50 at the end of this month to begin negotiations, has targeted a "good deal" within the two-year timeframe.
The failure to agree a deal would see Britain forced to adopt so-called 'WTO rules' with punishing tariffs until a new arrangement was finalised.
"WTO rules do not provide as good a trade regime as a bilateral deal, like the one that we should or could have between the EU and the UK," Mr Lamy said.
He played down Mrs May's 'no deal' pledge as an attempt to flex Britain's negotiation muscles that was typical of the tough talk before trade deals.
"I've been in trade negotiations for 25 years of my life and I've heard this notion that no deal is better than a bad deal probably about 500 times," he said.
Mr Lamy said the government's hope of walking away with a strong trade deal after the two-year exit negotiations was simply unrealistic.
"These agreements are long to negotiate," he said.
"An ambitious trade agreement - like the one we should have been the United Kingdom and the European Union - will probably take four, five, six years to negotiate."