Theresa May kicked off her keynote speech to the Conservative conference with a joke about Boris Johnson's ability to "stay on message".

Giving her keynote address to the conference for the first time as Prime Minister, Ms May said: "When we came to Birmingham this week, some big questions were hanging in the air. Do we have a plan for Brexit? We do. 

"Are we ready for the effort it will take to see it through? We are."

Smiling, she asked: "Can Boris Johnson stay on message for a full four days?"

"Just about," she answered, after applause and laughter from the audience.

Earlier in the conference, Mr Johnson was ambushed by a Sky News reporter who asked him to sign a giant cheque made out to the NHS for £350m.

The Foreign Secretary refused to sign the cheque, giving his reason as: "Because you’re doing a pointless stunt, that’s why."

Mr Johnson was also criticised after referring to Africa, the second-largest continent in the world, as "that country".

His suggestion that it would be a "fantastic thing" if the UK had a "more sensible system" for dealing with migration between Australia and the UK was also dismissed by Amber Rudd, the Home Secretary.