Theresa May has hit back at Boris Johnson's plan for Brexit, claiming it would "tear up" the UK's pledge to Northern Ireland.
In a series of interviews on Tuesday afternoon, May tore into Johnson's Brexit vision as it would see the need for customs checks on goods crossing the UK's border with Ireland.
Her comments came after Johnson launched a stinging attack on the Prime Minister in a highly-anticipated speech at a fringe event at the Conservative Party conference on Tuesday.
The former foreign secretary claimed May's so-called Chequers proposal – which would see the UK follow Brussels' rules on goods and agri-foods – was "dangerous and unstable".
To a standing ovation he urged the PM to "chuck Chequers" and push forward with a free trade deal with the EU.
Speaking to the BBC, May said: "Well there are one or two things that Boris said that I am cross about. He wanted to tear up our guarantee to the people of Northern Ireland.
"Northern Ireland is part of the United Kingdom. You know we are all, he and I, all members here are members of the Conservative and Unionist Party.
That's because we believe in the union of the United Kingdom. Northern Ireland is part of that union. And we have a guarantee for the people of Northern Ireland and we are upholding that."
May has repeatedly claimed there would be no hard border between the UK and Ireland after Brexit.
However, in an interview with the BBC's Andrew Marr on Sunday she was unable to guarantee there would not be customs checks in the case of 'no deal'.
World Trade Organisation rules, which the UK would have to abide by if there was no agreement with Brussels, require border checks.
When May was reminded of this, she said: “As a United Kingdom government we remain committed to doing everything we can to ensure no hard border between Northern Ireland and Ireland."