He, and the former Brexit minister David Davis, are pushing for a Canada-style free trade deal with the European Union - a proposal May says will split Northern Ireland from mainland Britain by making the British province adhere to different customs rules.
In his latest outburst, the former foreign secretary questioned whether the prime minister believes in Brexit as he rejected her Chequers strategy for leaving the European Union.
She said: 'Well to be honest, I've four weeks to go until my due date so I think rather than worrying about how the child will affect the job I'm anxious about how the job will affect the child because those are my priorities at the moment, making sure this one comes into the world with ten fingers, ten toes is top of my list'.
"My message to my party is let's come together and get the best deal for Britain", May told the BBC in the central English city of Birmingham.
"We want to capture that spirit for a new generation, celebrate our nation's diversity and talent, and mark this moment of national renewal with a once-in-a-generation celebration", May said. "You can't beat (Jeremy) Corbyn by becoming Corbyn".
Ms Davidson dismissed the attack on the Premier as she condemned Mr Johnson.
May has shown little sign of shifting away from her Chequers plan, named after her country residence where she hashed out an agreement on Brexit with her ministers in July, despite growing criticism that her proposals offer the worst of all worlds.
Since losing her parliamentary majority in a disastrous snap election past year, May has faced endless internal plotting and rumours of a leadership challenge. "I think it's got to wait until after Brexit", said Alan Dixon, 72, from Derby.
Jon Trickett, shadow cabinet office minister, said: "How can we trust this Tory Government with our country's security when they can't even build a conference app that keeps the data of their members, MPs and others attending safe and secure?"
It was an inauspicious start, the year after May's closing conference speech was marred by a collapsing set and a prankster stage invasion.
Corbyn presented a radical economic programme for government at his own conference last week.
The Conservatives are slightly ahead of Labour in opinion polls, but a slew of senior figures have warned they need new ideas.
May announced plans on Sunday to increase the property purchase tax for foreign buyers, amid concerns overseas investors are driving up prices, with the money to be used to tackle homelessness.