Boris Johnson has accused the European Union of "moral blackmail" over the Northern Ireland backstop as he insisted he would never allow the return of a hard border with the Republic.
In the latest Tory leadership hustings held in Belfast, both Mr Johnson and his rival Jeremy Hunt insisted the backstop had to go if there was to be a Brexit deal with Brussels.
Mr Johnson told party members he had felt so strongly over the issue he had resigned from Theresa May's Cabinet over the the Chequers plan which included the backstop.
"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," he 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.
"The way to protect the Union is to come out the EU whole and entire. Solve the border issues where they belong in the FTA (free trade agreement) we are going to do."
Mr Hunt, the Foreign Secretary, said that he also had opposed the backstop in Cabinet but had not made his misgivings public.
He said he opposed the measure, intended to ensure there is no return of a hard border on the island of Ireland, because it meant the UK could trapped in the EU customs regime indefinitely.
"I do recognise that we are never going to have a deal to leave the EU with the backstop. So it has to change or it has to go," he said.
"I was one of the people who argued against accepting the backstop in the Cabinet, but I think it is important the prime minister has a loyal foreign secretary so I kept those discussions private.
"But that has to change and we have to find a different solution."