There is "absolute clarity" on how NI-GB trade will work under the Brexit deal, Health Secretary Matt Hancock has said, despite Boris Johnson appearing to contradict earlier outlined plans.
Mr Hancock said the details would be worked out in a future EU trade deal.
The PM was accused of not understanding his deal after he briefed Conservative members in Northern Ireland.
He said firms could "bin" customs forms as there will be no checks or barriers on trade across the Irish Sea.
That contradicted Brexit Secretary Steve Barclay, who said in September that businesses in Northern Ireland would have to submit exit declaration forms after he initially denied that was the case.
On Saturday, Mr Hancock told BBC Breakfast there is "absolute clarity" between what the prime minister and Mr Barclay said.
"It's highly technical because it's about the types of checks and agreement but, crucially, all of this can be properly addressed in the future trade agreement talks because we don't want to have any tariffs with the EU and, as far as I can see, they don't want to have any with us."
The health secretary added that the way in which customs declarations will work "will be determined by the agreement that we strike".
"For instance, we don't want to have any tariffs with the EU, that is a matter for the future relationship and the prime minister is talking about how we're going to negotiate this, which is not yet agreed.
"What is agreed is how we leave."
On Friday, Labour's Shadow Brexit Secretary Sir Keir Starmer tweeted that Mr Johnson's remarks to Conservative Party members in Northern Ireland suggested he "either doesn't understand the deal he has negotiated or he isn't telling the truth".
Boris Johnson either doesn’t understand the deal he has negotiated or he isn’t telling the truth. Probably both. pic.twitter.com/fvqh7PWPjS— Keir Starmer (@Keir_Starmer) November 8, 2019
In a video of the meeting, which has appeared on social media, businessman Irwin Armstrong asked Mr Johnson if he could tell his staff "we will not be filling in any customs declarations for good leaving Northern Ireland to go to GB".
Mr Johnson replied: "You can."
He added: "If somebody asks you to do that, tell them to ring up the prime minister and I will direct them to throw that form in the bin.
"There will be no forms, no checks, no barriers of any kind - you will have unfettered access."
On Friday, broadcasters questioned Mr Johnson about his remarks.
He told reporters: "Northern Ireland and the rest of GB are part of the UK customs territory and there can be no checks between goods operating in one customs territory.
"We're the UK - we will not be instituting such checks."
Mr Armstrong, who said his company makes small shipments to pharmacies in the rest of the UK, was not sure Mr Johnson was being "absolutely serious in his answer".