Former justice secretary David Gauke says a Conservative majority at the upcoming election would be a "bad outcome for the country".
Mr Gauke - who confirmed he will run as an independent in 12 December poll - was among the MPs expelled from the Tories by Boris Johnson after he voted against a no-deal Brexit.
He said a majority led by Mr Johnson would mean a "very hard Brexit".
But Tory Minister Michael Gove said his former colleague was "precisely wrong".
Mr Gove told BBC Breakfast the Conservatives were pursuing "a good Brexit deal which works for whole UK [and] which will enable us to have a relationship with the EU based on free trade and friendly co-operation."
Responding to Mr Gauke's comments, Mr Johnson said: "We are fighting for every vote we can get. I regret we haven't got his support, but we will do our best in the campaign ahead."
Mr Gauke confirmed his decision to stand in South West Hertfordshire - where he has been the MP since 2005 - at a political awards ceremony on Tuesday.
But he has urged voters in some constituencies to vote for the Liberal Democrats
Speaking to BBC Radio 4's Today programme, Mr Gauke attacked the policy of the Conservatives to not extend the implementation period for Brexit past December 2020.
During these months, the UK would stick to the EU rules on issues such as freedom of movement.
The Tories plan to negotiate a free trade agreement with the European Union during that time, but have pledged to leave without one if no deal is reached by the deadline.
Brexit Party leader Nigel Farage cited the pledge as one of the reasons for his decision not to stand candidates in the 317 seats won by the Tories at the last general election, in 2017.
'Disastrous for prosperity'
Mr Gauke said "one simply cannot renegotiate a trade deal in that time period", and leaving without a deal would be "disastrous for the prosperity of our country… [making] whole sectors unviable".
But he said Mr Johnson was so "boxed in" to the plan that he couldn't change his mind even if he wanted to - and he showed no sign of that.
"He would have letters flooding in to the chairman of the 1922 committee [trying to oust him] and Nigel Farage would be out making a lot of noise," said Mr Gauke.
"I don't think that either the parliamentary party or the wider Conservative membership would allow him to do that. He is boxed in unless Parliament is in a position to force him to extend."
Leaving the IP on WTO terms would be devastating to many sectors of our economy. It’s a thoroughly irresponsible policy.— David Gauke (@DavidGauke) November 12, 2019
There are many excellent Conservative candidates who I wish well but a Conservative majority at the next election would be a bad outcome for the country.
Mr Gauke said his comments were not a personal attack on Mr Johnson, although he said the PM "lacks qualities some would ideally want in prime minister".
But he urged voters to support "the centre ground" in the election so they could stop a hard Brexit, even lending their support to the Liberal Democrats if needs be.
"I have to say I am impressed by [Lib Dem leader] Jo Swinson and if I was living in a lot of constituencies I would lend my vote," he told Today.
Mr Gauke now backs a confirmatory referendum on Mr Johnson's deal versus Remain.
"I have reluctantly come to that view," he said. "I thought the best outcome for our country was for us to unite behind some kind of soft Brexit [but] that option isn't there any more. The country is too polarised and there isn't the support for it.
"[Mr Johnson's plan] is a harder Brexit than what was promised to the British people in 2016.
"Because the consequences of the Johnson deal are so significant, we do need to check back in with the people, and it is perfectly possible to get a parliamentary majority for that after the election."
Just four months ago David Gauke was a cabinet minister and regarded as one of the safest pair of hands in the Tory Party.
He is now urging voters to stop Boris Johnson from winning a majority.
His decision to stand as an independent candidate is prompted by his fear that Mr Johnson is "boxed in" to a no-deal Brexit by his refusal to consider any extension of the transition period beyond December 2020.
An impossible timetable, Mr Gauke believes, in which to secure a trade deal - and a view shared by many hard line Brexiteers.
Mr Gauke is one of only a small band of former Tory rebels who've chosen to fight on, rather than to quit politics altogether.
But Lib Dem sources said they were unlikely to stand aside in his Hertfordshire seat.
Meanwhile, Downing Street has shrugged off his decision and later Mr Johnson will repeat his Brexit message - that his deal is the only way to get Britain out of the rut and end the "groundhoggery".
Chancellor of the Duchy of Lancaster, Mr Gove, said his former colleague was "a good friend, but I think on this issue he's got it precisely wrong".
He told Breakfast: "The only way that we can Brexit done is by making sure we do have a functioning majority government.
"We're going to get a good deal with the EU and we're going to get it by the end of 2020."
Mr Gove added: "One of the problems that we've had is that Parliament has engaged in endless dither and delay on this, and that's because we haven't had a strong majority."
Earlier, Gagan Mohindra was chosen as the Conservative candidate for Mr Gauke's constituency.
Mr Mohindra is a member of Essex County Council and Epping Forest District Council.
Some parties are yet to choose their candidates for South West Hertfordshire, but Tom Pashby has been selected for the Green Party and Sally Symington will represent the Liberal Democrats.
Mr Gauke is not the first politician to call on the public to back a rival party in the December election.
On Monday, his fellow former Tory MP Nick Boles launched a scathing attack on both Mr Johnson and Mr Corbyn in the Evening Standard, and said people should vote Lib Dem.
This came after two former Labour MPs - Ian Austin and John Woodcock - said the electorate should back Mr Johnson as Mr Corbyn was "completely unfit" to be PM.