© Denis Balibouse/Reuters (L)/Toby Melville/Reuters (R)
Johnson, who dropped out of May's cabinet in July over her Brexit plan, slammed May's suggestion that the UK and EU could collect each other's tariffs as "entirely preposterous," in an interview with the Sunday Times.
The former top brass also argued that a Brexiteer politician would have got a better deal for Britain out of the negotiations, pointing to how he campaigned to leave while May was a staunch remainer.
Comment: Trying to take a page from Trump's book, Boris?
"Unlike the prime minister I campaigned for Brexit," he told the paper.
"Unlike the prime minister I fought for this, I believe in it, I think it's the right thing for our country and I think that what is happening now is, alas, not what people were promised in 2016," he added.
Laying out his stall for leadership, he also pointed to how he had been twice elected mayor of London and could bring that same success to the rest of the country.
Tackling the criticism of her plan on the first day of the conference, May warned her Chequers detractors to "stop playing politics" and urged Tory members to "come together" to back the Brexit plan.
However, May not only has to convince Brexiteers within her own party over the virtues of the Chequers blueprint, but the EU as well.
The bloc's leaders expressed skepticism over the Chequers plan at a summit in Salzburg earlier this month, with EU Council President Donald Tusk warning that it risked "undermining the single market."
Rowing in behind Tusk, EU leaders agreed that while progress over Brexit had been made on negotiating a new deal, they insisted that Chequers was not enough to reach an agreement.
However, a Brexiteer prime minister would also face an EU in no mood to back down. French President Emmanuel Macron previously called the leaders of the Brexit campaign "liars" who abandoned the UK citizens after the vote.
He added that the EU would "never" accept any Brexit deal which would put the EU's integrity at risk.
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