Former Labour MP Ian Austin has urged voters to back Boris Johnson, saying Jeremy Corbyn as Prime Minister is "too big a risk".
The long-time critic of the Labour leader spoke out as the two main parties sought to focus the election campaign on the economy with shadow chancellor John McDonnell pledging an "irreversible shift" in wealth in favour of working people and the Tories warning Labour's "fantasy economics" would wreck the recovery.
Mr Austin, who is not contesting the Dudley North seat he has held since 2005, told the Express & Star newspaper: "I must do everything I can to stop Jeremy Corbyn from getting into power."
Mr Austin quit Labour in February in response to what he claimed was a "culture of extremism, anti-Semitism and intolerance".
He said "decent patriotic Labour voters" should vote Tory to help Mr Johnson get the majority he needs to stop Mr Corbyn from entering Number 10, adding that Mr Corbyn was "not fit to lead".
His intervention came just hours after Labour deputy leader Tom Watson announced he is standing down, saying the time was right for him to step away from politics.
In his first speech of the General Election campaign, Mr McDonnell will say that if Labour gain power on December 12, they will deliver a programme of investment "on a scale never seen before in this country".
However his Conservative counterpart Sajid Javid will accuse Mr McDonnell and Jeremy Corbyn of behaving like "anti-vaxxers" on the economy.