British Prime Minister Theresa May said she had not watched a speech on Tuesday by her former foreign minister Boris Johnson, preferring instead to talk about her party that she described as being in "really good heart".

To standing ovations, cheers and laughter, Johnson, May's most powerful critic in her party, said in his speech her so-called Chequers plan to leave the European Union was a "cheat".

Asked whether she was cross with Johnson, who stole the show at her Conservative Party's annual conference, May told BBC television: "there are one of two things that Boris said that I'm cross about ... He wants to tear up our guarantee to the people of Northern Ireland".

Showing little sign of considering changes to her Brexit blueprint, May said it was only her Chequers plan that could uphold the unity of the United Kingdom and declined to speculate on whether she could see Johnson as a future prime minister.

"I'm not speculating about or commenting about jobs for individual members of parliament. This is not about the jobs of politicians, this is about the jobs of people out there in our country, it is about protecting those jobs for the future."

In a separate interview with Sky News, she added that: "Boris always puts on a good show but what matters to people is what we are delivering for them on the things that affect their day to day lives."

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