Boris Johnson's plan to agree a trade deal with Brussels by the end of next year is "unrealistic", according to Michel Barnier.
The EU's top Brexit negotiator was in a private meeting with MEPs when he made the comments, a recording of which was passed to The Independent.
Johnson has insisted that the UK will be able to reach a free trade agreement with the EU in time for the post-Brexit implementation period ending on 31 December next year.
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But Barnier said: "With regards to this agreement, we will not get everything done in 11 months. We will do all we can – we won’t do it all.
He continued: “It is unrealistic that a global negotiation can be done in 11 months, so we can't do it all. We will do all we can to get what I call the 'vital minimum' to establish a relationship with the UK if that is the time scale."
The Conservative election manifesto said the prime minister will not seek an extension to the implementation period, meaning the UK would leave the EU without a long-term trade arrangement at the end of 2020.
Barnier said: "If there is no extension, we will still have a few months to achieve what I would describe as the minimum necessary for the economy and security or to prepare for a cliff-edge.
"Either we have a trade agreement or we will be in the WTO framework. We can’t take contingency measures on trade to give us more time … If there’s no deal, it is a cliff edge for trade."
A Conservative spokesman said: "Barnier also said the withdrawal agreement couldn’t be reopened and the backstop would never be removed.
"The prime minister secured a new deal that abolished the backstop. Today we are fighting for every vote because only a majority conservative government will get Brexit done."
But Labour’s Andrew Gwynne said Barnier's comments showed the prime minister was “playing the British public for fools”.
"Johnson’s sell-out Brexit deal will put us on the fast-track to a cliff-edge no deal Brexit. And that’s exactly what he wants, so he can drive the UK into a toxic trade deal with Trump and put the NHS up for sale to US drug companies.
"The only place Johnson’s sell-out Brexit deal should go is into the deep freeze, just like Johnson when he doesn’t have the guts to face a reporter."
For the Lib Dems, Tom Brake said: "Michel Barnier’s comments are further proof that Boris Johnson’s key campaign slogan is just another Tory fib. Just like his claim that there’ll be no checks on goods to Northern Ireland and his lies to the Queen, Johnson can’t help but mislead the British public.
"His reckless Brexit approach will send the country careering off the no-deal cliff next year."