Boris Johnson vows to deliver Brexit 'no ifs or buts' as he enters Number 10

Speaking outside 10 Downing Street, the Tory leader said he would meet the October 31 deadline "no ifs or buts".

Watched by girlfriend Carrie Symonds, Mr Johnson promised he would "change this country for the better".

Arriving in Downing Street after being invited by the Queen to form a government during an audience at Buckingham Palace, Mr Johnson vowed to prove the Brexit doubters wrong.

He said: "I am standing before you today, to tell you the British people, that those critics are wrong - the doubters, the doomsters, the gloomsters are going to get it wrong again."

He predicted that "the people who bet against Britain are going to lose their shirts because we are going to restore trust in our democracy".

He added: "And we are going to fulfil the repeated promises of Parliament to the people and come out of the EU on October 31, no ifs or buts.

"And we will do a new deal, a better deal that will maximise the opportunities of Brexit while allowing us to develop a new and exciting partnership with the rest of Europe based on free trade and mutual support.

"I have every confidence that in 99 days' time we will have cracked it.

"But you know what we aren't going to wait 99 days, because the British people have had enough of waiting.

"The time has come to act, to take decisions, to give strong leadership and to change this country for the better."

He promised action to fix the social care crisis, make the streets safe and improve the NHS.

He said: "I will take personal responsibility for the change I want to see.

"Never mind the backstop, the buck stops here."

But on the issue of the Irish border - the main stumbling block in reaching a Brexit deal - Mr Johnson said he was "convinced" a solution could be found without checks at the Irish border and without the "anti-democratic backstop".

He added: "It is of course vital at the same time that we prepare for the remote possibility that Brussels refuses any further to negotiate and we are forced to come out with no-deal.

"Not because we want that outcome, of course not, but because it is only common sense to prepare."

Mrs May had used her farewell address in Downing Street to urge Mr Johnson to secure a Brexit deal.

She said the "immediate priority" was "to complete our exit from the European Union in a way that works for the whole United Kingdom".