Boris Johnson has claimed there will be no border in the Irish Sea as a result of his Brexit deal.
Under the deal NI will continue to follow many EU rules on food and manufactured goods, while the rest of the UK will not.
NI will also continue to follow EU customs rules but will remain part of the UK's customs territory.
A government risk assessment says that will lead to new administration and checks on goods entering NI from GB.
Those new processes and checks are widely interpreted as amounting to "an Irish Sea border."
Mr Johnson was taking part in a BBC phone in and was questioned by a caller from Belfast.
He told her there would be no checks on any goods from NI to the rest of the UK.
He was asked if he could commit that NI businesses would not encounter additional paperwork or fees when dealing with GB.
He said: "I absolutely can. This is a matter for the UK government and we will make sure that businesses face no extra costs and no checks for stuff being exported from NI to GB."
However Mr Johnson gave no commitments on what would happen with GB to NI trade.
Under his deal means Northern Ireland will remain part of a "single regulatory zone" with the Republic of Ireland, a zone that will apply EU rules.
The EU has particularly strict rules on the importation of "products of animal origin" - meat, fish and dairy products.
Those products must enter the EU through a border inspection post where all shipments are subject to document checks and a high proportion are physically checked.
Products of animal origin from Great Britain entering Northern Ireland would be subject to these checks.
A few countries, such as New Zealand, have a deal with the EU where only 1% of consignments are checked.
It is possible that the UK could negotiate a similar deal but it would not be able to eliminate checks entirely unless the whole of the UK was going to stay in the single market.
The current political declaration, which sets out the broad shape of the future EU-UK relationship, suggests that is unlikely.
Mr Johnson said that if the deal was found not to be working for the people of Northern Ireland then the Stormont Assembly can vote to leave that regulatory zone.
The caller pointed out that Stormont has not sat for over 1,000 days - Mr Johnson said this was 'a great shame.'
The Prime Minister also said he was "1,000,000% committed" to maintaining the union between Northern Ireland and Great Britain.