After May bowed to pressure on Friday and announced she would resign as Tory leader within two weeks, justice secretary David Gauke and international development secretary Rory Stewart condemned Johnsons readiness to embrace a no-deal, saying it would be hugely damaging to the national interest.
The move, part of a concerted anti-Johnson push by opponents of a hard Brexit, followed comments by the former foreign secretary on Friday, soon after Mays resignation speech in Downing Street, that the UK would definitely leave the EU deal or no deal on 31 October if he became leader in July.
The remark infuriated the soft-Brexit wing of the party, with some MPs and ministers even warning that there would be serious numbers of moderate Conservatives who would be ready to vote down a Johnson government if he set the country on a path to no deal.
In a clear attack on Johnson, Gauke, writing in todays Observer, warns that candidates who fail to acknowledge the enormously harmful effects of crashing out of the EU will fuel populism and risk doing untold harm to the economy and national interest.
All those that do have such aspirations have a responsibility to set out their approach to Brexit, which is anchored in the hard realities of the situation. We should not pretend that leaving the European Union without a deal will be anything other than enormously harmful to our economy, weaken our security relationships and threaten the integrity of the union, said Gauke.
His comments come ahead of the release of European election results, which are expected to show Nigel Farages Brexit party trouncing the Conservatives.
There is too often reluctance in pointing out the likely outcome of no deal. The pretence by people who should know better that no deal is somehow manageable or could be addressed simply by proper preparation has only encouraged a growing part of the population to be unwilling to make any kind of accommodation with the EU. Loose talk about no deal has helped give credibility to the simplistic slogans of the Brexit party.
The warnings by Gauke expected to be endorsed by the chancellor, Philip Hammond, in media interviews came after Stewart, who has declared his intention to stand, tore into Johnson and said he would refuse to serve in a government under his leadership.
Stewart said: I spoke to Boris, I suppose, about two weeks ago and I thought at the time he had assured me that he wouldnt push for a no-deal Brexit. So, we had a conversation about 20, 25 minutes and I left the room reassured by him that he wouldnt do this.
But it now seems that he is coming out for a no-deal Brexit. I think it would be a huge mistake. Damaging, unnecessary, and I think also dishonest.