Two Northern Ireland MPs have told Europe's chief Brexit negotiator there is "significant opposition" to the UK Government's refusal to consider extending the timetable for talks.
DLP leader Colum Eastwood and Alliance's Stephen Farry have co-signed a letter to Michel Barnier supporting an extension to the transition period.
It comes amid speculation that Labour could push for the deadline to be extended beyond December after insisting the Government should not rush trade talks.
Shadow Cabinet Office minister Rachel Reeves said the Government is rightly focusing on the Covid-19 pandemic and must "take the time that is needed" to agree future trading arrangements with the EU.
In a joint letter the Westminster leaders of the SNP, Plaid Cymru, Liberal Democrats and Green Party - plus Mr Eastwood and Mr Farry - warn of "significant opposition to the UK Government's extreme position amongst the business community, the general public and elected representatives".
Mr Farry stated: "This is not about Remain v Leave, but rather about getting things right.
"The end of December deadline was always going to be very challenging, but it is next to impossible during this Covid-19 crisis."
He added: "The bandwidth of the business community is already fully taken, with no space for proper Brexit planning.
"Any decision to seek an extension must be taken by the UK Government before the end of June. Time is of the essence."
UK law currently prevents a minister extending the transition period.
Talks between the UK and EU appear to have made little progress with both sides expressing their dissatisfaction.
Cabinet Office Minister Michael Gove challenged the EU to show "flexibility", including over the issue of fishing in UK waters.
For Labour, Ms Reeves said the party "absolutely do not want" the UK to exit the transition period in December without a deal.
She told Sky News' Sophy Ridge On Sunday: "I would say to the Government the most important thing is we get a good deal, not any deal, but the best deal we can have.
"The last thing our country and our economy needs at the moment is a further shock that could put jobs and livelihoods at risk.
"So, don't rush this. All of the attention of Government at the moment is on fighting the coronavirus, that is the right thing, don't rush this, take the time that is needed.
"But at the moment the Government is saying we can still do this by the end of the year and we need to hold them to account to getting not just any deal, but the best deal we can, by the end of this year."
Ms Reeves said the Government must outline a new timetable if it cannot secure a good Brexit trade deal by the end of the year.
Asked how the talks were going, Mr Gove told the BBC's Andrew Marr Show: "Well, but with one proviso.
"The proviso is there is a big philosophical difference between the position that we take and the European Commission take.
"The European Commission want us to follow the rules even though we have left the club and the European Commission want to have the same access to our fish as they had when we were in the EU even though we are out.
"The challenge for the EU is to show just a little bit of their fabled flexibility."
Responding to the joint letter, the UK Government said it will not ask to extend the transition period, and if the EU asks it will say no.