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 prime minister said companies 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".
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."
'Just being bombastic?'
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."
'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, 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."
At the same event on Thursday night, Mr Johnson also told Conservative members Northern Ireland had "got a great deal".
He added: "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."
'Many questions about cross-Irish Sea trade'
Analysis: Chris Page, BBC News Ireland correspondent
Trade experts say they believe there will need to be checks on goods moving from Great Britain to Northern Ireland - even if they are not destined to cross the Irish border.
That is because under the Brexit deal Northern Ireland will have to follow some of the rules of the European single market - on food produce, for example.
There are strict regulations on "products of animal origin", which must enter the EU through a border inspection post where goods are frequently checked.
These issues may be very technical but in the context of Brexit and the election they are also highly political - and not just in Northern Ireland, where they are going to matter the most in practical terms.
BBC News NI's economics and business editor John Campbell said it was "not completely clear" what Mr Johnson meant by "free movement" in that context.
"Free movement gives all citizens of EU countries the right to travel, live and work wherever they wish within the EU," he added.
"The government has promised to end free movement and this will also apply in Northern Ireland.
"It seems that Mr Johnson was referring to the continuation of the Common Travel Area, which allows passport-free travel between the UK and Ireland and gives British and Irish citizens a range of rights in each other's countries."
The Liberal Democrats criticised the prime minister after the comments for wanting to treat Northern Ireland differently to the rest of the UK.
The party's Brexit spokesman Tom Brake told PA Media: "The single market and freedom of movement are a great deal - even Boris Johnson recognises this.
"So why isn't he keeping them for the whole of the UK as part of the many benefits of EU membership?
"It is clear that the best deal for the UK is the one we have now - in the EU," he said.