The DUP's Sammy Wilson told the Prime Minister that the Port of Larne in Northern Ireland has been told to prepare to become a border control point.
In the Commons during Prime Minister's Questions, the East Antim MP produced a letter which had been sent to the port's authorities telling them to prepare for border controls and said a piece of land was being considered for vehicles to park.
"The Prime Minister has stated that when we leave the EU at the end of this year Northern Ireland will still remain a full part of the United Kingdom," Mr Wilson said.
"Yet I have in my hand a letter received by the management of the Port of Larne only this week stating that they have to prepare to become a border control post and fourteen acres has been looked at for car parking or for lorry parking and for construction."
He added: "Can the Prime Minister explain how Northern Ireland can remain a full part of the United Kingdom if people coming from the rest of the UK into Northern Ireland have to pass through a border control post and would he advise the management to tear this letter up as well?"
Boris Johnson said he had not seen the letter.
"But I can tell him absolutely, categorically that there will be no new customs infrastructure," the Prime Minister said, "for the very simple reason that under the protocol it is absolutely clear in black and white that Northern Ireland is part of the customs territory of the whole of the United Kingdom and will be joining the whole of the United Kingdom in our new independent trade policy and doing free trade deals around the world."
Earlier during Northern Ireland questions DUP Westminster leader Sir Jeffrey Donaldson asked if the Government was seeking a waiver from the EU to prevent the need for customs declarations on goods shipped between GB and NI and how advanced any discussions were.
Brandon Lewis replied: "We do continue to take forward discussions on the implementation of the Protocol... Provisions must include the minimum-possible bureaucratic consequences for businesses and traders and we will respect what we have promised, which is unfettered access."
He added: "There should be no tariffs in internal UK trade because the UK is a single customs territory."
Tory chair of the Northern Ireland Affairs Select Committee Simon Hoare said: "(He) knows that NI business wants to prepare to make Brexit a success, the problem is that they don't quite know what they are preparing for... Is he persuaded of the merits of providing stepping stones between now and December 31 so that business knows what to prepare for and in what time frame?"
Mr Lewis said: "We will set out further details to help businesses prepare for the end of the transition period at the earliest appropriate moment.. further guidance will be published this summer to make sure that people and businesses know what they need to do to prepare for the end of the transition period which will be at the end of December this year."