The South Staffordshire MP, who played ‘kingmaker’ in Theresa May’s successful rise to Number 10 in 2016, said Mr Johnson was “the only person who can deliver Brexit and defeat Labour”.
Mr Williamson has taken on a leading role in Mr Johnson’s leadership campaign since he was sacked as defence secretary by Mrs May over the Huawei leaks scandal.
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In his first public endorsement of the former foreign secretary, Mr Williamson told the Express & Star: “I think he’s the best candidate.
"He’s the one who can deliver change for both the Conservative Party and the country.
“I will be enthusiastically backing him and very much hope I can play a small role in making sure that his name is the one that the party chooses as leader.
“The bottom line is that the only person who can deliver Brexit and defeat Labour is Boris Johnson."
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Mr Williamson's announcement of support came as calls for Mrs May to stand down reached fever pitch, with many Tory MPs furious at her revamped Brexit deal.
They include Walsall North MP Eddie Hughes, who said it was “time for someone new” to lead the country, and Lichfield MP Michael Fabricant, who urged her to go “in the next few days” so a successor can be in place by late July.
Commons Leader Andrea Leadsom quit the cabinet on the eve of the European elections on Wednesday, while a series of Cabinet ministers have asked for meetings with Mrs May to raise their concerns about the Withdrawal Agreement Bill.
The Prime Minister will meet Sir Graham Brady, the representative of Tory backbench MPs, on Friday for a meeting which could seal her fate.
She has previously agreed to set out the timetable for the contest to replace her after a crunch vote on her Brexit deal, widely expected on June 7.
And Mr Williamson believes former London mayor Mr Johnson is the man to take over.
"He reaches out to a lot of people, and I think people in the Black Country and Staffordshire can see that he wants to deliver what was voted for in the EU referendum, as well as changing the Conservative Party for the better," Mr Williamson said.
“He has a very bold vision for the country and very much wants to see the opportunities that Brexit can present realised.
“There is no doubt in my mind that he is a man who can beat Labour. Both of want to make sure that we stop Jeremy Corbyn from getting into Downing Street.”
Mr Johnson confirmed he was mounting a leadership bid last week.
He is currently seen as the front runner, ahead of a lengthy list of candidates including Dominic Raab, Jeremy Hunt, Michael Gove, Sajid Javid and Penny Mordaunt.
Former Chief Whip Mr Williamson played a major role in the aftermath of the 2017 snap general election in sealing the deal with the DUP that kept Mrs May in No 10.
He insists he has "always got on well" with Mr Johnson, although sources say the pair have not always seen eye-to-eye.
Mr Williamson is said to have "privately vowed" to stop Mr Johnson's leadership bid in 2016, which eventually hit the buffers due to Mr Gove's last minute entry into the race.
'No chance of passing'
Mr Williamson said he shared the concerns of MPs, saying the Prime Minister would be best advised to shelve plans for a vote on the deal in order to prevent another heavy defeat.
“From what I have heard the Prime Minister say, I am very concerned about what is being proposed,” he said.
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“I have not yet seen a copy of the bill, but am approaching it with a great deal of scepticism in terms of whether it can deliver what the public voted for in the EU referendum.
“I can’t see it having the slightest chance of passing through Parliament.
“The most logical thing for the Prime Minister to do would be to pull it. There is no point in inflicting a defeat on yourself that has been obvious for quite a long time.”
It came as polls opened across the West Midlands in the European elections, with Nigel Farage’s Brexit Party widely expected to land a significant number of seats.
The results are due to be declared on Sunday night.