Michel Barnier, the EU's chief Brexit negotiator, has told Britain the economic coronavirus fallout has made a new trade deal vital to both sides.
He was speaking as EU and UK negotiators enter a fourth and final round of talks this week that could determine if a comprehensive new agreement can be struck by the year-end deadline.
Britain formally left the EU nations last January but still largely operates as if it were a member, thanks to a standstill grace period which expires on December 31.
Boris Johnson has vowed not to seek any negotiating extension which might be on offer from the EU and, since such an extension must be sought before June 30, hopes for any new deal look bleak.
Mr Barnier yesterday told the UK's 'Sunday Times' that London and Brussels could not afford to make the economic situation even worse by ending nearly 50 years of associating without a new deal.
"If we don't get an agreement then that will have even more consequences. And then of course those will be added to the already very serious consequences of the coronavirus crisis," he said.
The previous round of talks in May ended in mutual recriminations and the coronavirus, which has reduced talks to videolink only, has proved another obstacle.
Further delays were caused by Mr Barnier and his UK counterpart, David Frost, being forced into isolation after each catching the virus.
Mr Johnson is expected to discuss the issue with EU chiefs Ursula von der Leyen and Charles Michel at a video summit on June 19, just days before the extension application deadline.
His team argues the point of Brexit was to give Britain the right to set its own rules - but Mr Barnier says the UK is seeking single market benefits without fair competition.
Yesterday, Mr Barnier accused UK negotiators of reneging on the commitments Mr Johnson signed up to in a non-binding political declaration that accompanied the sides' formal divorce deal agreed late last year.
"The UK has been taking a step back - two steps back, three steps back - from the original commitments," the EU negotiator told the 'Sunday Times'.
Meanwhile, a House of Lords report has warned that businesses in Britain could decide it is economically unviable to continue operations in Northern Ireland unless flexibility is shown in the EU negotiations.
The North will have to follow EU rules on agriculture and manufactured goods, ensuring access to its single market and keeping the Border with the Republic of Ireland free-flowing in a key concession maintaining a decades-old peace. The group of peers said: "The combination of uncertainty, lack of momentum and lack of time, compounded by the shock of the Covid-19 pandemic, is a potent threat to economic prosperity and political stability in Northern Ireland."
The mechanism ensuring the Border remains open after the Brexit transition period finishes at the end of this year is known as the Northern Ireland Protocol and was agreed between the EU and UK during the Brexit talks.
The UK government published a document attempting to flesh out some detail in May.
It said checks will be needed on some goods entering Northern Ireland from the UK as part of Brexit but bureaucracy will be kept to a "minimum".
The Lords European Union committee's report said: "The command paper's heavy reliance on the future tense underlines how little progress has been made thus far, how many issues remain to be resolved, and how much work still needs to be done before the protocol becomes operational on January 1, 2021."
Officials overseeing the process need to be flexible, it said.