Britain's Rishi Sunak, who lost the race to become Britain's prime minister on Monday, said the party must now unite behind winner Liz Truss.
"It's right we now unite behind the new PM, Liz Truss, as she steers the country through difficult times," Sunak said on Twitter.
Truss, 47, will be only the UK's third female prime minister following Theresa May and Margaret Thatcher.
She will formally take office on Tuesday, after Johnson tenders his resignation to Queen Elizabeth II.
"I know she has the right plan to tackle the cost-of-living crisis, unite our party and continue the great work of uniting and levelling up our country," Johnson tweeted.
"Now is the time for all Conservatives to get behind her 100 per cent."
The Tory winner faces "the worst in-tray for a new prime minister since Thatcher", The Sunday Times wrote.
Millions say that with energy bills set to rise by 80 percent from October -- and even higher from January -- they face a painful choice between eating and heating this winter, according to surveys.
The Times and Daily Telegraph newspapers reported Monday that Truss was considering freezing energy bills for consumers, with the government reimbursing suppliers.
On Tuesday morning, Johnson will deliver a farewell speech at Downing Street before flying to Scotland -- where heavy rain is forecast -- to hand his resignation to the queen.
Truss is expected to fly separately to accept the queen's invitation to form a new government, to ensure continuity of government in case of any mishaps.
On her return to Downing Street, the new prime minister will then give a short address to the nation. By tradition, that happens on the steps of Number 10.
But it may have to be moved indoors with forecasts for thundery downpours, matching Britain's dismal outlook as the Truss government starts life.
(Agencies)
Curator Emma Slocombe with ikat robes and arabachi coats in the Peter the Great Room in Blickling Hall Amit Roy 
View of the entrance to Blickling Hall on a rainy autumn dayAmit Roy 
Kente cloth from Ghana in the Long Gallery in Blickling Hall 
Decoration in the Great Hall at Blickling Hall Amit Roy 
Palampore fragment made in coastal south-east India in the Brown Room in Blickling Hall Amit Roy 
Bed valance dating from early 1700s in the Italian Print Room at Blickling Hall Amit Roy 
Indian inspired fabric draped on sofas in the South Drawing Room in Blickling Hall Amit Roy 
Indian inspired fabric draped on sofas in the South Drawing Room with Indian carpet in Blickling Hall Amit Roy 
Karun Thakar has picked out the geometric patterns in the bookcase and the Kente cloth in the Long Gallery in Blickling Hall Amit Roy 
Large tapestry depicting Peter the Great defeating the Swedish army at the Battle of Poltava in 1709 Amit Roy 
ikat robes and arabachi coats in the Peter the Great Room in Blickling Hall Amit Roy 
Baghs and kanthas made by women provide contrast to the men only portraits in the Great Hall at Blickling Hall Amit Roy 
Baghs and kanthas in the entrance Great Hall at Blickling Hall Amit Roy
Baghs and Kanthas in the Great Hall in harmony with stained glass window at Blickling Hall Amit Roy 
Kimonos reflect Japanese influence in Upper Ante Room in Blickling Hall Amit Roy 
Kimono in Upper Ante Room in Blickling Hall 
A view of the Palace and fort at Agra shows romantic side of India Amit Roy 
Blickling Hall Amit Roy 
Table carpet, European 1670-1680 Amit Roy 
Unfinished 17th century Italian embroidery in the Print Room at Blickling Hall Amit Roy 
Picture of Kashmir shows a romantic view of India Amit Roy 
A panel of Ashburnham bed hangings made in Gujarat for sale in Europe in the Brown Room in Blickling Hall Amit Roy 
Colcha made in the 1600s by women in Bengal for the Portuguese market in the Long Gallery at Blickling Hall Amit Roy
