Writing exclusively for The Independent, Nicky Morgan accused Mr Johnson of a “dereliction” of duty and said his endless currying of favour with hardline Brexiteers is leading Jeremy Corbyn to Downing Street and the Conservatives to “our undoing as a party”.
Ms Morgan, who chairs the influential Treasury Committee, hit out at Mr Johnson for setting out his own Brexit “red lines”, which she argued diverged from Ms May’s plans and showed no regard for the financial security of millions of Leave backing voters.
Her attack comes as Mr Johnson faces a broader Tory backlash from MPs and ministers following an interview in The Sun that further stoked speculation about his leadership ambitions, as he set out his Brexit demands and strayed well outside his Foreign Office brief.
Former education secretary Ms Morgan penned her piece following its publication, saying: “The UK Government should be focusing on getting the UK out of the EU in the least damaging way, not debating arbitrary red lines set down to try to curry favour with those who want a utopian ultra-free trade, low tax, minimal regulation state.
“Those who are pushing this agenda have no place in a responsible government – it is a dereliction of the duty to act in the national interest. And it has to stop.”
She argued Mr Johnson appeared to see Ms May’s recent speech in Florence as a compromise which “takes him and ardent Brexiteers away…from the low tax, minimal regulation state” they want.
She added: “Those who think that Brexit offers an opportunity to move to some low tax, almost off-shore de-regulatory haven don’t seem to care about the threat posed by Corbyn. They need to wake up.
“Not only is there no appetite for their vision amongst the electorate, it isn’t smart economics and would be our undoing as a party.”
The backlash also emerged elsewhere, with Tory MP Simon Hart taking to Twitter in the wake of Mr Johnson’s interview to say: “Have I missed something or has Boris Johnson suddenly been given some new role and authority in all this?”
A cabinet minister told The Independent Mr Johnson is becoming increasingly isolated on the frontbench because he is destabilising his party and country for the sake of his own career, in the hope he can win a future leadership battle with the support of Brexit backing Tories.
On Saturday Scottish Conservative leader Ruth Davidson also attacked Brexiteers who she said are “selling people short” by being over-optimistic about withdrawal, and said policy on should be left to “serious” people.
But Mr Johnson will not have missed polling showing his high profile interventions over Brexit have launched him into first position in a recent poll of favourites for next leader among party members, who have the deciding say.
He is insisting any Brexit transition period must not last "a second more" than two years, that the UK should not abide by new EU rules during that time and Britain must not make payments to Brussels after it, adding that there can be “no monkeying around”.
He even used his interview on Saturday to stray into the territory of Chancellor Philip Hammond, his Cabinet rival in the Brexit debate, to demand that the minimum wage rise faster than it is due to.
Mr Johnson’s intervention came as a letter by leading Brexiteers demanded Ms May abandon negotiations with Brussels if the EU does not agree to talk about a future trade deal by Christmas.
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