Boris Johnson's pledge to strike a trade deal with the EU by the end of next year has been branded "unrealistic" by Brussels' chief Brexit negotiator.
During the general election campaign, the prime minister said he thought he could "absolutely guarantee" an agreement on the UK's future relationship with the EU could be delivered in 11 months, if Brexit happens on 31 January next year.
But Michel Barnier claimed in a meeting with MEPs that "we won't do it all" by the deadline.
"It is unrealistic that a global negotiation can be done in 11 months, so we can't do it all," the EU chief Brexit negotiator is heard saying in an audio recording obtained by the Independent website.
"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."
If the UK leaves the EU at the end of January under the terms of Mr Johnson's Brexit deal, the country will enter a "transition period" until the end of 2020.
This is hoped to minimise changes for citizens and businesses while leaders hammer out the details of a trade deal, due to kick in at the end of 2020.
The prime minister has vowed not to extend the 11-month transition period.
If no trade agreement is reached by the end of the transition period, Mr Barnier warned there would effectively be a no-deal separation and the UK would revert to base trade rules as set by the World Trade Organisation.
In the leaked recording, Mr Barnier added that if Mr Johnson keeps his word and does not request an extension by the end of June next year, then there would be some time to prepare for a "cliff-edge" divorce.
"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," he is heard saying.
The leaked comments came on the final day of general election campaigning ahead of Thursday's vote.
A Tory source told Sky News: "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."
Labour responded to Mr Barnier's leaked comments by accusing Mr Johnson of "playing the British public for fools".
Referring to the prime minister walking into a freezer as a reporter tried to question him earlier in the day, Labour's shadow communities secretary Andrew Gwynne said: "Boris Johnson is playing the British public for fools.
"He's hiding in fridges to dodge interviews precisely because his fake Brexit slogans can't stand up to scrutiny, just like his empty words on the NHS.
"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."
Liberal Democrat Brexit spokesperson Tom Brake claimed Mr Johnson's "reckless Brexit approach will send the country careering off the no-deal cliff next year".
He added: "Voting Liberal Democrat tomorrow is your chance to stop Boris Johnson, stop Brexit and build a brighter future within the EU."