Hardline Brexiteer Boris Johnson hasn't stayed quiet in the months after his resignation as Britain's foreign secretary. And his latest scathing criticism of Prime Minister Theresa May is fueling speculation he will eventually mount a leadership challenge. 

Johnson, writing in the Daily Telegraph, compared the negotiating plan that led to his resignation as May's foreign minister to entering battle under a "white flag" of surrender to the EU.

Reuters political correspondent Will James said the stakes were especially high for the prime minister at the moment because Brexit is so close.

"It's less than seven months away now and that means that everything that is undecided that's putting pressure upon her position has to be decided within a very short space of time. The question is what are the terms of that departure and, of course, will she still be in charge when we do."

Johnson's regular columns have kept him in the public eye.

This one was timed to coincide with parliament's return from summer recess this week.

Downing Street issued a rebuttal claiming the strategy the party adopted was the only credible one out there.

The public acknowledgment of Johnson's attack may be evidence of how legitimate a threat he and other party rebels are.

The European Union isn't happy with Prime Minister May either. 

"If you're on the European Commission's side, what they see is Britain trying to cherrypick access to the single market -- the best bits of the single market. That's something that if they let Britain do that, who else is going to ask next? So for Theresa May she's really plotted a path inbetween those two factions and is now feeling the force of their anger on both sides," James said. 

But for Brussels May's departure could be disastrous.

Johnson's on record as suggesting the UK should negotiate more like Donald Trump.

And Trump, he said, would "go in bloody hard."