Jeremy Corbyn has claimed the Tories are running a Government in "disarray" and Labour now represent the political mainstream.
The Labour leader told The Guardian the 4,000 word article written by Boris Johnson for The Daily Telegraph last weekend was a "lapse of discipline".
He insisted it would not have happened in his shadow cabinet, which he said was more united in the wake of June's election result.
The Labour Party now represents the British political mainstream, Corbyn added, because the Conservatives were "lacking direction."
The veteran left-winger vowed to keep up the pressure on the Tories by "challenging them at every step."
Corbyn's comments come ahead of the party's first conference since the election, and a key Brexit address to be made in Florence by Theresa May.
The Prime Minister is due to map out her vision for Brexit today (Friday September 22) while Labour's conference begins on Sunday.