The deadline for the end of the Brexit transition period should have been extended due to the global pandemic, former Irish Taoiseach Bertie Ahern said.
Continued uncertainty would be a retrograde step and the last thing either side needs, he added.
He told Times Radio: "One of the huge mistakes of this year, in my view, particularly when the pandemic hit, was to insist that the transition period ended on the 31st of the 12th.
"It is a known fact in the history of trade deals that trade deals are prolonged.
"They create division and difficulty and the idea of trying to push a trade deal into a calendar year in the middle of a world pandemic was - to put it at its mildest - not a good idea, and it still isn't."
He said British and EU negotiators could decide on Sunday that they needed more time.
"I think people should be big enough to say: 'Listen, we've had a pandemic all year, let's have extra time on this.'
"The third thing is they square the circles and they say: 'Listen, there's not a lot of difference here,' and they both compromise."
Mr Ahern added: "I think to just explode it all and to go into a whole period of uncertainty and retrograde steps for Europe, retrograde steps for the United Kingdom, is the last thing they should do."
He said some in Scotland, Wales and Northern Ireland would use a no-deal Brexit to articulate their case for some form of independence and some kind of change.
"I think in the case of Northern Ireland the good news is for customs and trade, Northern Ireland by and large stay in the Single Market and stay in the customs agreements, with some complications.
"At least the agreement that was made before Christmas last year looks as if it stands, so that's a good thing.
"So maybe Northern Ireland won't be as difficult as it could have been."