Northern Ireland needs Brexit clarity from a government "consumed" by the coronavirus emergency, the Belfast civil servant in charge of the country's preparations said.
Two scenarios have been drawn up at Stormont depending on whether or not the UK secures a comprehensive free trade agreement with the EU.
Without one, the border protocol surrounding compliance with aspects of the Republic's system is expected to apply, senior official Andrew McCormick said.
He said: "Time is tight but what is needed from London is clarity on these issues."
Civil servants are assuming neither the UK nor Europe will request an extension of the one-year transition agreement due to run out at the end of the year.
The director general of international relations for the devolved administration gave evidence to a Stormont committee of Assembly members yesterday.
Admitting the focus had been on combating the coronavirus infection, he said: "The UK Government has also been consumed by, the bandwidth for activity has been dominated by the virus."
He added: "There is no lack of pressure from us at official level and then engagements at ministerial level pushing the UK Government for more clarity and resolution of the issues that matter to us."
SDLP MLA Colin McGrath labelled plans to stick to the end-of-year deadline as "ludicrous".
Mr McGrath, chairman of Stormont's Executive Office committee, said the government has refused to "shed its ideological fervour" and believes it will not be able to cope with dealing with Brexit as well as the pandemic.
"It is ludicrous that in these circumstances, the British Government refuses to shed its ideological fervour and admit that it is not possible to construct a new economic relationship with Europe in the short time left," he told yesterday's committee meeting. "Government bandwidth is stretched to its limit dealing with the public health emergency we're facing.
"I have no confidence that it can sustain the pressure of Brexit negotiations on top of that," he added.
A UK Government spokesperson said "the Prime Minister has made clear he has no intention of changing" the transition period deadline, adding: "We have worked closely with the devolved administrations at every step of this process and will continue to keep them updated on the progress of negotiations, to ensure we have a future relationship that works for all parts of the UK."