Boris Johnson has vowed to take personal responsibility for delivering Brexit, saying “never mind the backstop, the buck stops here”.
In a speech bound to sting his predecessor, the new prime minister said he would come up with a new Brexit deal and disprove “the doubters, the doomsters and the gloomsters”.
Delivering his first address from outside Number 10, Mr Johnson said: “I will take personal responsibility for the change I want to see.
“It is time we unleashed the productive power of every corner of England, Scotland, Wales and Northern Ireland. The awesome foursome incarnated in that red, white and blue flag.”
He claimed no-deal remained a “remote” possibility and added: “The British people are tired of waiting. It is time to act.”
Watched by his girlfriend Carrie Symonds, Mr Johnson reassured EU citizens around the country.
“Thank you for your contribution to our society,” he said. “Under this government you will have the absolute certainty to live here and remain.”
He also promised work would begin to recruit 20,000 police officers, upgrade 20 new hospitals, fix the social care “crisis” and “level up” funding for primary and secondary schools.
Earlier, Mr Johnson officially gained the keys to Number 10 after meeting the Queen at Buckingham Palace.
He had an audience with the head of state for around 15 minutes.
The exact shape of Mr Johnson’s new top team is not yet known, but he has said there will be more women and people from ethnic minority backgrounds in his cabinet.
Priti Patel, Alok Sharma, Rishi Sunak, Oliver Dowden, Tracey Crouch and Robert Jeninck have all been tipped for promotion.
But Mr Johnson’s rival in the Tory leadership race, Foreign Secretary Jeremy Hunt, is understood to be fighting for his political future.
Sky News understands he was offered a demotion to defence secretary, and has been in a stand off with the new administration over his future role.
It is expected a woman will be appointed to one of the great offices of state – the foreign secretary, home secretary and chancellor – following Mr Johnson’s promise to do so in a TV debate last week.
Brexit is likely to be the centrepiece of his work the first 100 days – with the UK’s EU departure due to happen on 31 October.
The incoming prime minister has pledged to take Britain out of the EU “do or die” on that date, with or without a withdrawal agreement.
He has described as dead a key part of it known as the Northern Ireland backstop – an insurance policy to stop a hard border reforming on the island of Ireland.
Asked what changes Brussels could offer to make it acceptable, he told a hustings: “No to time limits or unilateral escape hatches or all these elaborate devices, glosses, codicils and so on that you could apply to the backstop.”
But Mr Johnson’s Brexit plans could face fierce opposition from Tory MPs who want to block him pursuing a no-deal divorce.
Around 47 of them voted against Conservative orders to limit the next prime minister’s ability to close parliament to force through no-deal.