London: Boris Johnson officially became Britain's new Prime Minister on Wednesday and promised to leave the European Union on October 31 "no ifs, no buts" under a "new deal" with the 28-member economic bloc.
The 55-year-old former foreign secretary and London Mayor met Queen Elizabeth II, who asked him to form a new administration, Buckingham Palace said in a statement.
A picture released by the palace showed Johnson shaking hands with the 93-year-old monarch and bowing.
Speaking outside Downing Street after his appointment by the Queen, Johnson said he wanted to "change the country for the better".
He said the UK would leave the EU on October 31 "no ifs, no buts".
"The buck stops with me," he added.
The newly appointed Prime Minister said "his job is to serve the people".
Theresa May, who was forced to resign amid a mounting rebellion from within her party over her Brexit strategy that failed to clear the Parliament votes threshold three times, earlier in the day formally tendered her resignation to the Queen.
As the face of hard Brexit in the ruling Conservative Party, Johnson has pledged to get Britain out of the European Union (EU) within the deadline of October 31 - with or without a deal.
The prospect will prove divisive as the Opposition Labour Party and pro-EU Liberal Democrats have already voiced plans to bring down his government if he attempts a drastic no-deal exit from the EU.
Labour leader Jeremy Corbyn has demanded that Johnson call a General Election and promised to table a motion of no confidence in him "when it is appropriate to do so".
The MPs from within Johnson's own party remain vehemently opposed to a no-deal Brexit, with Chancellor Philip Hammond, justice secretary David Gauke and international development secretary Rory Stewart among some of the Cabinet members who have already declared their defiance by refusing to serve in his Cabinet.
Johnson is expected to spend his first few hours at No. 10 Downing Street finalising the top jobs in his Cabinet, with Indian-origin MP Priti Patel widely tipped to be back in the frontline after she had been forced to resign from May's Cabinet as international development secretary in 2017.
The 47-year-old ardent Brexiteer declined to "speculate" over Johnson's plans but said she was confident the new Prime Minister had plans for a "diverse" Cabinet, given his track record of deploying a diverse team as the Mayor of London.
"It is important that the Cabinet should represent modern Britain as well as a modern Conservative Party," said Patel, a prominent member of the "Back Boris" leadership campaign.
Fellow Brexiteer Rishi Sunak, the son-in-law of Infosys co-founder Narayana Murthy, and housing minister Alok Sharma are also in line for frontline jobs in what Indian-origin peer Lord Jitesh Gadhia has hailed as the most diverse Cabinet in British history.
"Johnson will appoint the most ethnically diverse Cabinet in British history. This is a big moment - and a bold statement - by the incoming Prime Minister," said Gadhia.
Johnson, the UK's third Prime Minister since the June 2016 referendum in favour of Brexit, will face some of the same challenges as in the path of May, including a very slim parliamentary majority and a reliance on the support of the Democratic Unionist Party of Northern Ireland in order to govern.
The DUP has been one of the central critics of the controversial Irish backstop - a post-Brexit insurance policy against a hard border between EU member-country Ireland and UK territory Northern Ireland.
It has been rejected by Brexiteers as a way of keeping the UK tied to EU rules even after Brexit, ultimately resulting in the collapse of May's premiership.
The former Mayor of London, who has in the past described himself as a son-in-law of India by virtue of his now estranged wife Marina Wheeler's Indian mother - Dip Kaur, has stressed that the UK's relationship with India must run deeper than just trade.
In a letter earlier this month, Johnson had played up his personal relationship with Prime Minister (Narendra) Modi as he promised to deliver a "truly special UK-India relationship".
If I am elected Conservative Party Leader and Prime Minister, I will work closely with our friends in the Indian Government, business and society to deliver a truly special UK-India relationship, he had said.