The UK will have to settle for a trade deal with the EU similar to Canada, Michel Barnier has warned.
The EU's lead negotiator suggested the UK's decision to leave the single market and customs union meant any deal would "have to work on a model that is closer to the agreement signed with Canada".
Theresa May had ruled out a Canada-style deal in her Florence speech last month.
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Mr Barnier also claimed it would be "several years" before a Brexit trade deal was agreed - another blow to Mrs May, who told the House of Commons on Monday that the UK would "need to know" the terms of its future relationship before any transition period was fully agreed.
Speaking to French newspaper Les Echoes, Mr Barnier said a transitional period would "leave us more time to prepare for the future relationship", as he raised the prospect of Brexit talks lasting well beyond the UK's formal departure.
"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."
On whether a transition deal was now a certainty, Mr Barnier said: "If we reach an agreement on the orderly withdrawal of the UK, such a period, both short and framed, is possible.
"To my mind, it makes sense that it covers the financial period, so until 2020."
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Brexit talks are currently deadlocked as the EU looks to extract a "firm and concrete commitment" from the UK on a financial settlement, with Mr Barnier refusing to start talks on trade until a divorce bill is agreed.
However, EU leaders have started preparatory internal discussions on a future relationship with the UK.
Commenting on Mr Barnier's remarks, the Prime Minister's official spokesman insisted it was still the Government's position an EU-UK trade deal could be agreed within the next 12 months.
He said: "We are confident of achieving that deal, and we come at this from a unique position in terms of our existing relationship with the EU, and that we can achieve that trade deal quickly."