PSNI Chief Constable Simon Byrne has told British Prime Minister Boris Johnson that it would not be possible to police the 300 crossing points of the border with his current number of police officers.
Simon Byrne said his 30-minute conversation with the prime minister happened by video call last Friday.
"We were face to face on a video call for over half an hour," he said.
"It was a very open conversation trying to tell him we saw that it was nigh on impossible to try and police over 300 crossings with the amount of police officers we had.
"It was a candid conversation, he was responsive to what we said and at the end of the day, how it landed and what he thought... you're going to have to ask him."
During a meeting of the Northern Ireland Policing Board earlier today Mr Byrne said he had "made it clear" to the Northern Ireland Office that police will not "staff any form of border security" after the UK leaves the EU.
"We have been working closely with the Northern Ireland Office (NIO) to understand our constitutional position in relation to any proposed changes to border arrangements, and I have been clear with the NIO in particular it will not be the role of the PSNI to staff any form of border security.
"We are clearly there to facilitate normality and day to day policing, but not to carry out custom checks and the function of other agencies in whatever proposal is or isn't agreed in the next few weeks and indeed I have taken legal advice on that basis to confirm to me the independence of the office of Chief Constable and the duties I have to make sure that police officers are used for legitimate policing purpose."
