Exclusive: shadow foreign secretary says she warned party leaders going to polls would be ‘act of catastrophic political folly’

Emily Thornberry has declared she is entering the race to succeed Jeremy Corbyn, revealing she warned the Labour leadership that backing a Brexit election would be an “act of catastrophic political folly”.

The shadow foreign secretary set out her pitch to be the next Labour leader in an article for the Guardian, arguing she has already “pummelled” Boris Johnson across the dispatch box and knows how to exploit his failings.

Directly addressing those calling for a new leader from northern England, she made the case that Corbyn’s replacement should be chosen on the basis of their “political nous and strategic vision”, rather than where they come from or their views on Brexit.

Her declaration makes her the first candidate to confirm she wants to be in the race, although Lisa Nandy, Keir Starmer, Clive Lewis and David Lammy have indicated they are seriously considering it.

Rebecca Long-Bailey, who is backed by a number of Corbyn allies, is highly likely to stand, along with Jess Phillips and possibly Yvette Cooper.

In her article, Thornberry sought to demonstrate her political judgment by revealing that she sent a lengthy written note to the leadership before the election, explaining it would be disastrous for Labour to go to the polls before Brexit was settled in a referendum.

She likened the party’s decision to back an election to “crackers voting for Christmas” because it gave Johnson a chance to fight an election on a subject of his choosing at a time of his choosing.

“Listening to Labour colleagues on the media over the last week, I have repeatedly heard the refrain that the problem we faced last Thursday was that ‘this became the Brexit election’. To which I can only say I look forward to their tweets of shock when next Wednesday’s lunch features turkey and brussels sprouts … I wrote to the leader’s office warning it would be ‘an act of catastrophic political folly’ to vote for the election, and set out a lengthy draft narrative explaining why we should not go along with it.

“I argued that the single issue of Brexit should not be enough to give Johnson a five-year mandate to enact his agenda on every issue. Instead, I said we should insist on a referendum on his proposed deal, to get the issue of Brexit out of the way before any general election.”

She blamed pro-leave colleagues in the shadow cabinet for having insisted on backing the election that Johnson wanted, which resulted in Labour losing a net total of 59 seats.

Corbyn told the Guardian at the outset of the campaign that he had made the decision to support an election himself. Describing a shadow cabinet meeting, he said, “I put it to them quite clearly: I said, our objections are now gone. We are now supporting a general election – and everybody gulped. I didn’t alert anybody in advance – it was my decision. On my own. I made that decision. And they gulped, and said, Yes Jeremy.”

Since last week’s disastrous result, some Labour MPs have called for the next leader to be someone from the north or Midlands who is not strongly pro-remain to help win back trust in Labour heartlands that went Tory in some cases for the first time.

Caroline Flint, who lost her Don Valley seat, has said Long-Bailey and Nandy are the only candidates worth considering for that reason and accused Thornberry of insulting voters in the north, which has caused Thornberry to threaten to sue her for “making shit up”.

Addressing such concerns, Thornberry, who is MP for Islington South, said: “The question is how we fight back from here. The answer is certainly not to have some great ideological debate between left, right and centre. Neither is it to set this up as a battle between leave or remain, north or south, and indeed men or women. When did we stop being for the many, not just half of us?”

She said the goal should be strategic thinking and a proper plan for the future rather than simply aiming to earn raucous cheers at a rally.

Describing her time opposite Johnson as his shadow while he was foreign secretary, Thorberry claimed she “took the fight to him every day and pummelled him every week” so that his mask slipped to expose a “mendacious, lazy, dangerous charlatan, unable to hide behind the tiresome smokescreen of bluster he usually relies on”.

“So when the Labour leadership contest begins, whoever is standing – and I hope to be one of the candidates – the first question shouldn’t be about their position on Brexit, or where they live in our country,” she said.

“The first question should instead be: what’s your plan for taking on Boris Johnson over the next five years? And do you have the political nous and strategic vision to reunite our party, rebuild our machine, gain the trust of the public, give hope to our declining towns and smaller cities, and never again waste the opportunity to take back power?”

She said the prize was being able to focus on Labour’s priorities over more than a century of helping the elderly, housing, the unemployed, and alleviating child poverty.

Starmer, a fellow north London MP, set out his pitch for the leadership in the Guardian on Tuesday, without confirming he would definitely run.

He called for his party not to lurch to the right as a result of last week’s devastating election result and acknowledged Labour did not do enough to tackle the Conservatives’ central election pledge to “get Brexit done” nor deal sufficiently with antisemitism.

Both Thornberry and Starmer have served in Corbyn’s shadow cabinet, alongside Long-Bailey. In contrast, Nandy has been on the backbenches since she resigned in 2016 when Corbyn’s leadership was challenged.

She set out her thinking in the Observer last weekend, arguing Labour had “lost touch with the day-to-day lived experience” of the people it wished to represent and calling for it to reconnect with the parts of the country that “have come adrift” to regain voters’ trust.

Phillips also told the Observer: “Belief matters, and we failed the test. That is an existential problem for the party that is named for working people.”

The leadership contest will start formally in January and last until Corbyn steps down at the end of March.