Mr Davis made his remarks as Mr Johnson vied with rival Jeremy Hunt for the leadership of the party, with the result due to be announced on July 23. Writing on the Conservative Home website, he said: “I’ve announced my support for Boris Johnson, because I really feel this is a last chance for the Conservative Party. “The anger on the doorsteps about the Government’s failure to deliver Brexit is tangible.
“There is now a serious danger of the Conservative Party being eclipsed by the Brexit Party, and the prospect of the most left-wing Government ever under Jeremy Corbyn becomes ever more realistic and frightening.
“Until we tackle Brexit, we cannot move on and unite and renew our Party. Only then can we defeat Corbyn.
“I am backing Johnson because he will deliver Brexit, unite our Party and then defeat Jeremy Corbyn. It’s as straightforward as that.”
Referring to his resignation a year ago, Mr Davis said Theresa May’s Government had “reached a fork in the road” during the explosive cabinet meeting at Chequers at which she unveiled the central tenets of what became know as the Chequers Plan.
He said: “It was a concession too far. A capitulation to the EU – and the end of what had been a serious negotiating stance.
“It gives me no pleasure to have been proven right. As I predicted, if we were not serious about leaving without a deal, then the EU would not offer us an acceptable deal. It may have seemed paradoxical to some, but anybody in business knows that you have to be prepared to walk away from a negotiation, and you have to mean it.”
The Chequers plan and subsequent withdrawal agreement (which has since been rejected three times by the House of Commons) were simply unacceptable “for reasons which are now well rehearsed”, Mr Davis said.
He explained: “The Irish backstop threatened the integrity of our union, we were handing a vast quantity of UK taxpayers’ money over to the EU with very few guarantees in return, and the UK was still subject to the jurisdiction of the European Court of Justice.
“Johnson has a clear plan to leave the EU, and as a former Brexit Secretary I can say that it is credible and has my support.
“He will seek a new withdrawal agreement which will deliver a more favourable terms for the UK. May’s Government was wrong to propose to hand over £39billion before receiving any guarantees as to a future deal, and Johnson will ensure that payments to the EU are only handed over until there is greater clarity concerning the future relationship between the UK and EU.
“Alternatively, we can look at an interim agreement on tariffs until a more permanent deal is settled.
“Finally, and crucially, he is absolutely correct to state that, come what may, the UK will leave the EU on October 31.”
Such a stance was vital to Britain’s negotiating position, as well as being the right thing to do because it was what the British public wanted and demanded, Mr Davis said.
He added: “It is in everybody’s interests to agree a way forward which is fair and lasting.
“As Angela Merkel has said: “Where there’s a will, there’s a way.”
“Boris Johnson has the plan, the energy and the conviction to ensure Brexit occurs. This will unite and benefit our country and the Conservative Party.
“Then we can move on to defeating our real opponent – Jeremy Corbyn.”