The UK Government has shown a complete disregard for Northern Ireland's economy by not seeking to extend the Brexit transition period, the Deputy First Minister has said.
Michelle O'Neill said the decision announced yesterday by Cabinet minister Michael Gove was "shameful".
However, First Minister Arlene Foster said it came as "no surprise".
A senior retail figure, meanwhile, called for more clarity on the way ahead.
Aodhan Connolly said companies and consumers need to know some details now, particularly on mitigating the costs of any friction in trade between Northern Ireland and the rest of the UK.
He said an already herculean task had got harder.
In a tweet yesterday Mr Gove said he has officially given notice to EU negotiators that the UK Government will not request an extension beyond the December 31 deadline.
His decision was made during a meeting of the EU joint committee.
UK sources were keen to depict the meeting as the last formal opportunity to request an extension, as it was the final scheduled meeting of the joint committee before the July 1 deadline to make such a request.
Later Scottish and Welsh ministers decided not to take part in a Brexit conference call with the UK Government in protest at Mr Gove's actions.
Mrs O'Neill, speaking after a meeting of the British Government-EU joint committee on the implementation of the Northern Ireland Protocol, described the announcement as "careless and shameful".
"It is completely careless and shameful for the British Government not to seek an extension to the transition period and it shows complete disregard for our local economy, which is facing a huge shock as a result of the Covid-19 pandemic," she said.
Speaking to RTE, Mrs Foster said it was no surprise the Government had stuck to its stated position.
"That was the position they were elected on last December, that they would get Brexit done, that there wouldn't be any extensions," she said.
"I've been saying that talking to colleagues in the Executive for quite some time.
"So, not really surprised, I think what we really need to see happening now is clarity for our businesses."
UUP leader Steve Aiken said he was not surprised by the announcement.
He said that four years since the Brexit vote there is still no detail on the plan, including provisions to offset the risk of additional costs of goods coming in from the rest of the UK, deal or no deal.
Mr Aiken said there has been no risk assessment of the implications of additional costs of trade with the "biggest market, that is the rest of the country".
But he added: "The trade talks are probably now reaching the crunch point and that's what we are seeing."
Alliance MP Stephen Farry described the announcement as "utterly reckless".
Mr Connolly, director of the Northern Ireland Retail Consortium, said: "It is not unexpected... but we have been saying since January that to have everything in place to protect Northern Ireland businesses and consumers it will be a herculean task, and that has only got harder."
Mr Connolly said businesses and consumers cannot afford the cost rises that will follow increased friction, adding that it takes time to properly put in new processes, new systems and to train people. "We need the detail now," he said.
Prime Minister Boris Johnson's spokesperson told RTE they are taking "a pragmatic and flexible approach" to try and help businesses in Northern Ireland adapt to the changes at the end of the transition period
Mr Gove also announced the UK will phase in import controls over a period of six months between Britain and the EU, a reversal of a previous position that they would be introduced immediately.
The UK Government has promised these will not apply to the flow of trade between Northern Ireland and the Republic, or between Northern Ireland and Britain.