Boris Johnson has been accused of not understanding his Brexit deal or what it means for businesses in Northern Ireland.
Labour criticised him after he briefed Conservative members on Thursday.
The PM said firms could "bin" customs forms because there would be "no barriers of any kind" to trade crossing the Irish Sea.
But that contradicts what Brexit Secretary Steve Barclay said last month about customs declarations for goods.
Mr Barclay said businesses in Northern Ireland would have to submit customs declaration forms after he initially denied that was the case.
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".
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."
'Just being bombastic?'
On Friday, broadcasters questioned Mr Johnson about his remarks.
He told 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."
'Tory campaign bogged down in Brexit detail'
Analysis: Jonathan Blake, BBC News political correspondent
Boris Johnson may have been trying to keep the focus of this campaign on his deceptively simple slogan of "get Brexit done" but three days in he's already found himself bogged down in the detail of his deal.
His words to Tory supporters in Northern Ireland on Thursday were an attempt to allay fears that businesses there would be subject to extra paperwork when shipping goods to the rest of the UK.
But with little detail - and confusing messages from ministers about how that aspect of his Brexit deal will work - the picture is unclear.
The prime minister will continue to present it as an agreement that is ready to go if he wins a majority at next month's general election.
Mr Armstrong, who said his company makes small shipments to pharmacies in the rest of the UK, said he was not sure Mr Johnson was being "absolutely serious in his answer".
The businessman told the PA Media news agency: "I want to believe him but is he just being bombastic and being Boris?
"I don't know Boris Johnson well enough, whether it's just what you say on a campaign trail or whether he is absolutely serious."
Mr Johnson told his Northern Ireland Conservative supporters: "Northern Ireland has got a great deal.
"You keep free movement, you keep access to the single market but you also have, as it says in the deal, unfettered access to GB."