Ulster Unionist leader Steve Aiken has said a border in the Irish Sea is now a certainty and he has asked the Government to urgently outline its plans to support Northern Ireland businesses.
It was revealed at Stormont earlier this week that physical posts were set to be installed at ports in Belfast, Larne and Warrenpoint.
Secretary of State Brandon Lewis told MPs he was determined there would not be an Irish Sea border and Northern Ireland would continue to have unfettered access to the UK market, but a number of local politicians do not believe that is possible.
SDLP MP Claire Hanna said that Arlene Foster and Michelle O'Neill must be assertive in demanding that London extend the transition period.
The details for the physical posts were revealed by Sinn Fein junior minister Declan Kearney during a committee meeting on Wednesday.
Prime Minister Boris Johnson had pledged that there would be no trade barriers between Northern Ireland and Britain.
But Mr Aiken said: "We now have it in the clearest of terms that there will be an Irish Sea border come January 1, 2021.
"The UUP had warned that this was sadly inevitable if we continued on the direction of travel set in December 2017 that the UK Government has allowed to continue.
"It is unforgivable that we have ended up here. For all the big talk and bluster, Northern Ireland will now have to deal with border control posts at our ports and airports.
"This places a huge burden on businesses here, who will be expected to be prepared for the changes in just over seven months amidst the crisis many are currently facing due to Covid-19.
"We urgently need to hear from the UK Government on how they are going to assist Northern Ireland in getting ready for this.
"We didn't ask for it, nor do we want it. We should not be expected to bear the cost. The Northern Ireland Executive must make a case for mitigation and for an urgent road map from HMG on how they are going to ensure that businesses in Northern Ireland are not disadvantaged by what they have agreed to."
Ms Hanna said: "After months of denying that there would be an Irish Sea border, the Government now must provide guidance to businesses on what to expect with these checks, which are happening in less than eight months.
"This is coming as businesses are fighting for their lives because of coronavirus. It is not a time for uncertainty.
"The First and Deputy First Ministers should be requesting that the transition period is extended. The Executive are not passengers in all of this. They are a player in the withdrawal agreement."
TUV leader Jim Allister said the revelation that the Government would put in place detailed plans with the Executive for physical posts at ports would present a "major test of their mettle" for unionist ministers.
"No unionist could agree to the imposition of such posts at Larne, Belfast and Warrenpoint. Not only would they represent a serious assault on our place within the UK, but they would create a barrier between us and our single biggest market in the rest of the UK," he said.
"Quite apart from the constitutional ramifications, it simply doesn't make economic sense."
Mr Lewis told the Northern Ireland Affairs Committee at Westminster that the Government remained "determined there will not be a border down the Irish Sea." He said: "We want to make sure there is unfettered access for Northern Ireland. Northern Ireland is part of the UK's single market. It is important it remains that way. It is important that we trade both ways."