Boris Johnson was at the centre of a vicious Tory row on the eve of the party conference after publicly attacking the Prime Minister over Brexit.

The former foreign secretary used a round of television interviews on Friday to lash out at Theresa May’s negotiating strategy, refusing to rule out a leadership challenge or voting against a Brexit deal.

His comments, which followed a lengthy newspaper article attacking the PM’s Chequers plan, was attacked by political opponents within the party.

Former education secretary Nicky Morgan told the PoliticsHome website that his alternative plan, dubbed “Super Canada”, was “”pie in the sky”, while Justice Secretary David Gauke warned that it could break up the United Kingdom if adopted.

However, his plan was defended by fellow hardline Brexiteers including Jacob Rees-Mogg, who likened the Prime Minister’s Chequers blueprint to the Charge of the Light Brigade, saying it was “a brave and mistaken dash against all the odds”.

The Conservative Party Conference is due to start in Birmingham on Sunday, with Mr Johnson one of the key attractions.

He is due to give a speech to a conference fringe on Tuesday – the day before the Prime Minister’s keynote address.

Speaking to the BBC on Friday he was asked repeatedly to rule out running against her.

He said: “The Prime Minister will go on, as she said to us herself, and as she said to the country, she’s a remarkable person, she will go on for as long as she feels it necessary.

“But the most important thing for me is to avert what I think would be a political and economic disaster for this country which is to agree to come out of the EU but still to be run by the EU, what is the point of that, what will we have done?

“And I think there is still time for her to change course.”

In a separate interview on Friday with Sky Mr Johnson had said that, despite the EU formally rejecting Chequers, its adoption would be “a political triumph for Brussels”.

The top Tory, in a Friday Telegraph op-ed, had accused the Government and civil service of a “pretty invertebrate performance” in negotiations and said there had been “a collapse of will by the British establishment to deliver on the mandate of the people”.

The former foreign secretary, who quit the Cabinet in July, argued for a new withdrawal agreement dubbed “Super Canada” which states that the Irish border question will be settled as part of the deal on the future economic arrangements.

Number 10 hit back at his proposal, with a source pointing out Mr Johnson had been part of the committee which agreed the need for a customs backstop in Northern Ireland.

David GaukeThe Justice Secretary David Gauke warned that Mr Johnson’s ‘Super Canada’ plan risked breaking up the United Kingdom (David Mirzoeff/PA)

Mr Gauke warned that a Canada-style deal could have grave consequences for the union.

He told the i newspaper: “If we end up with an arrangement whereby Great Britain leaves the single market and the customs union, and Northern Ireland stays in the single market and customs union, then over time Northern Ireland will become more and more integrated into the Irish Republic’s economy and less integrated into Great Britain’s economy.

“It’s hard to see how that doesn’t end in Northern Ireland leaving the UK.”

Mrs Morgan told PoliticsHome that Mr Johnson’s timing was deliberate, saying: “He knows how important the party conference is to the Prime Minister, to the party, and it’s obviously designed to make it clear that, yet again, we will be talking as much about Boris as we will about the Conservative programme for government next week.”

Dominic RaabBrexit Secretary Dominic Raab has accused the EU of politicising the Irish border (Kirsty O’Connor/PA)

In a separate development, Brexit Secretary Dominic Raab accused Brussels of using the Northern Ireland dispute as a way of penalising the UK for leaving the EU.

He told the Sun: “There are some out there in the Commission who see it as a lever beyond the substantive issue.

“There is no doubt there is a substantive issue about how we avoid a return to the hard border, but it has been magnified by those seeking to rely on it for political ends.”