Dominic Grieve, who served as attorney general, spoke out against Mr Johnson after Sarah Green overturned a huge Conservative majority to record a stunning victory at the Chesham and Amersham contest.
Mr Grieve, a remainer ousted from his Beaconsfield seat at the last general election, put the Tories defeat down to what he described as a "pretty sophisticated electorate" in the Buckinghamshire seat, which is located about a 35 minute drive from Watford in Greater London and not far from his former constituency.
Voters in Chesham and Amersham know "what a fraudulent prospectus is, and they have a very low opinion of the prime minister," Mr Grieve said.
"And they consider him to be a charlatan," he added. "And this is I think quite a widespread view among a certain section of the electorate that has consistently voted Conservative all their lives because this is a deeply conservative area with a 'small c'."
Thursday's by-election was triggered by the death of former cabinet minister Dame Cheryl Gillan, who took the seat with a majority of 16,233 in the 2019 general election, securing some 55 per cent of the vote.
In what was a big upset, the Lib Dem candidate Sarah Green secured 21,517 votes against Tory Peter Fleet's 13,489.
"Tonight the voice of Chesham and Amersham is unmistakable," Ms Green said in her acceptance speech. "Together we have said: 'Enough is enough, we will be heard, and this Government will listen.'"
The Greens came third and Labour in fourth, with its candidate securing just 622 votes in what was another hugely disappointing election result for the party. Labour was never expected to win but its poor showing will raise further questions about Keir Starmer's stewardship of the party.
Speaking to BBC News on Friday morning Mr Grieve, who was suspended from the Conservative Party in 2019 for defying the prime minister on a Brexit vote, claimed that the Chesham and Amersham was predictable.
"Indeed all the information I was picking up from the from friends who live there suggested to me that this was going to be the outcome,” he said.
The HS2 rail project and green belt planning concerns were two big issues in Chesham and Amersham contest - a seat that has only ever chosen a Tory MP. But Mr Grieve is adamant that Mr Johnson’s approach to politics and his "fraudulent prospectus" helped the Lib Dems win.
"I think this is quite a widespread view among a certain section of the electorate that has consistently voted Conservative all their lives because this is a deeply conservative area with a 'small C'," he said.
"And that's the fundamental problem the Conservative Party's got; they have a prime minister who appears to have an appeal to some sections of the electorate...Towards another group, who perhaps take life a bit more seriously, he comes across extremely badly and he's got much, much worse."
Conservative Party HQ has been attempting to play down the significance of the defeat, saying it is rare for governing parties to win byelection.
But policing minister Kit Malthouse branded the defeat "very disappointing" and said that there would have to be a "fairly significant post-match analysis" on what went wrong for the Tories.
Speaking to broadcasters earlier, Mr Malthouse said there was a "complicated message" after the humiliating defeat for the Conservatives.
Meanwhile, the Liberal Democrats have claimed their win will "send a shockwave through British politics". Party leader Ed Davey said that the result demonstrates that the "blue wall" of Tory southern seats could be vulnerable.
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