Agreement on a free trade deal with Britain after it leaves the European Union will take years to complete, the EU's chief negotiator has warned.
Michel Barnier said the discussions would be very different from the first phase of the negotiations on the terms of Britain's withdrawal and would entail "risks".
His comments came after Theresa May told MPs there could be no transition period after Britain leaves the EU unless there was agreement on a trade deal.
In a Commons statement, the Prime Minister said she remained confident that a deal was possible following the latest EU summit last week in Brussels.
However her remarks caused consternation among opposition MPs who warned the UK was heading for a "cliff-edge" break with the EU which businesses have been desperate to avoid.
Leaders of the other 27 EU member states agreed on Friday to begin discussions among themselves on a possible transition period - prompting hopes in Downing Street that they will agree to move on to trade talks in December.
In an interview with France's Les Echoes newspaper, Mr Barnier said that agreement on a time-limited transition period after Britain leaves the bloc in March 2019 should be possible.
"If we reach an agreement on the orderly withdrawal of the United Kingdom, such a period, both short and framed, is possible," he said.
"To my mind, it makes sense that it covers the financial period, so until 2020. It would leave us more time to prepare for the future relationship."
However he warned that the negotiations would continue to be "difficult" and that agreeing the future relationship between the EU and the UK would take time.
"The two phases are difficult. The second will be very different and will last several years," he said.
"It is truly unique because instead of promoting regulatory convergence, it will aim to frame a difference.
"It will involve risks, including about its political ratification, making all the more necessary transparency around these topics."
His comments raise new questions as to whether there can be a free trade deal in time for a transition period.
In the Commons, Mrs May said it was not possible to have a transition unless they knew what they were transitioning to.
"The point of the implementation period is to put in place the practical changes necessary to move to the future partnership," she said.
"In order to have that you need to know what the future partnership is going to be."