Mr. Johnson also says he will ensure the UK leaves the EU "undivided" if he becomes the British Prime Minister.
Boris Johnson says under "no circumstances" will there be a "hard border" in Ireland as a result of Brexit.
Speaking at a hustings in Belfast, the frontrunner to become the next British Prime Minister also says he will ensure the UK leaves the EU "undivided".
Mr Johnson accused the EU of moral blackmail over the backstop and says he's determined to leave by the October 31st deadline.
"Under no circumstances, whatever happens, will I allow the EU or anyone else to create any kind of division down the Irish Sea or attenuate our Union," Mr Johnson said.
"That is why I resigned over Chequers. It is a terrible moral blackmail it puts on the UK Government. We can't have that."
Mr Johnson told party members: "Of course we need to sort out the problems of the Northern Irish border, where those problems should be sorted out in the context of the free trade deal that we are going to do when we have left on October 31st.
"I think it's absolutely vital here in Northern Ireland to stress two things.
"Number one: that we will under no circumstances have a hard border. There will be no physical checks or infrastructure at the border in Northern Ireland.
"And number two: we will make sure we have an exit from the EU, a Brexit, that allows the whole UK to come out entire and undivided and we keep our union absolutely intact."
