Boris Johnson has given a speech full of energy and pomp as his final act as prime minister of the United Kingdom before heading to Scotland to formally offer his resignation to the Queen.

He urged his supporters to get behind his successor, Liz Truss, who will be appointed prime minister during her own audience with the Queen a short time after Johnson.

Truss was chosen on Monday to take over the top job in a leadership ballot against Rishi Sunak two months after Johnson announced his intention to step down in the wake of controversies that saw his party's support for him decline.

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Truss, who was foreign secretary under Johnson, had been widely tipped to win the ballot ahead of ex-treasury chief Sunak after winning the support of many Tories with a promise to roll back state intervention and slash taxes.

In the end, she convinced 81,326 of about 170,000 paying Tory party members to vote for her to Sunak's 60,399.

In his final address outside 10 Downing Street on Tuesday, Johnson was bombastic, unapologetically verbose, poetic and strange.

"This is it, folks," he said.

"The torch will finally be passed to a new Conservative leader.

"Through that lacquered black door, a new prime minister will shortly go to meet a group of fantastic public servants."

His final speech as prime minister had the feel of an election pitch rather than a swan song.

He pointed to the remarkable speed of Britain's vaccine rollout.

"That is government for you. That's this Conservative government."

Johnson energetically touted the country's future energy independence, particularly singing the praises of offshore wind power and nuclear reactors.

In two consecutive sentences, he likened himself to a booster rocket launching a spaceship out of the atmosphere, and to an ancient Roman statesman.

"I will now be gently re-entering the atmosphere, and splashing down in some obscure and remote corner of the Pacific," Johnson said.

"Like Cincinnatus, I will retire to my plough," he said.

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He thanked his family, including his dog Dylan and cat Larry, the chief mouser to the Cabinet Office.

"If Dylan and Larry can put aside their differences, then so can the Conservative party."

Larry has served under three prime ministers.