Ivan Rogers, who served as the UK's Permanent Representative to the EU, said that the country's departure from the bloc will be just the starting point in a rocky and uncertain journey that may continue for years.
Rather than having everything "boxed off" by the end of January, Rogers, while speaking to Xinhua news agency, said the negotiations could almost certainly stretch into the second half of third decade of the 21st century.
Johnson has repeatedly insisted that he will not extend the 11-month transition period, by the end of which the talks with Brussels on future relations are supposed to be concluded.
When Britain leaves the bloc at the end of January, the transition period will begin and its trading relationship with the EU will remain the same. It will continue to follow the EU's rules and contribute to its budget.
In an einterview with the French newspaper Les Echos, President of the European Commission Ursula von der Leyen said both sides needed to think seriously about whether the 2020 deadline was enough to reach an agreement.
Admitting that she was very worried about how little time was available, the President said: "It would be reasonable to evaluate the situation mid-year and then, if necessary, agree on extending the transition period."
If a trade deal was not agreed by December 2020, without agreed extension, it would leave Britain trading on World Trade Organization terms with the EU, with the likelihood of tariffs on imports and exports.
Commenting on the possibility of ending the negotiation and ratifying the trade deal in 2020, Rogers said: "There's an absolutely zero chance of this happening... We've already had three and a half years (since the 2016 EU referendum) and we haven't yet started the difficult stuff."
( With inputs from IANS )