Dominic Raab has described Boris Johnson as “wonderfully mercurial” and the only leadership candidate committed to leaving the EU by October 31.
Mr Raab, who was knocked out of the leadership race on Tuesday, told talkRADIO’s Julia Hartley-Brewer Mr Johnson was "the only one" who understood the importance of sticking to the deadline.
“We don’t give ourselves a shot in the next election if we are not out of the EU by the end of October. Boris is the only one who understands this," he said.
When asked whether Mr Johnson cared more about himself than Brexit, Mr Raab defended the frontrunner describing him as "a conviction Brexiteer".
“Boris is one of this wonderfully mercurial characters so there will always be people asking those kinds of questions,” he added.
“There are always uncertainties in politics and he has got to hold a coalition together - by which I mean a coalition within the Conservative Party."
Mr Johnson has topped all three rounds of voting, securing the votes of 143 of the 313 Tory MPs in the latest ballot on Wednesday.
The remaining four candidates - Mr Johnson, Sajid Javid, Michael Gove and Jeremy Hunt - are set to face two rounds of voting on Thursday to whittle down the contenders to the final two.
This comes after Rory Stewart was eliminated from the leadership race on Wednesday night.
Mr Raab said he admired Mr Stewart's “gumption”.
“I have known Rory since we came into Parliament in 2010 and I think people are interested in someone who is authentic,” he said.
“I disagree with him on quite a few policy issues - particularly on Brexit because I did not think his plan stacked up - but I am not going to bad-mouth any other candidate.
“It takes gumption to do what he has done and I admire him for it.”