Emily Thornberry warned that Jeremy Corbyn not having a public position on Brexit could affect their election chances months before voters went to the polls.
The shadow foreign secretary said in September while being filmed for a BBC documentary that she was worried about the Labour leader staying neutral.
She also feared not coming out strongly in favour of remain would hit the party hard at the ballot box.
Thornberry, who is expected to run as party leader, said she was “really pushing” at the time for Labour to openly back remain.
Labour suffered heavy losses in last week’s general election in its leave-voting former heartland seats, and it has been suggested voters were looking to the party to take a stronger position on leave, remain or a second referendum.
In an interview with the BBC’s political editor, Laura Kuenssberg, for her programme The Brexit Storm Continues during the party’s conference in Brighton, she made clear the extent of her doubts.
She said: “I think Jeremy is trying to find a compromise but if he goes into an election saying ‘I don’t have a view’ on the single biggest decision that we have to make – I think – what worries me is that every single interview he does will all be about Brexit.”
Asked if Labour could win an election with that position, Thornberry said: “Well, I think it makes it more difficult and that’s why I’m really pushing this because I want Jeremy in No 10.”
Labour’s plan was to renegotiate a Brexit deal with the EU within three months, then put it to a public vote within six months. Corbyn would then have decided at a special conference whether the party should back its own renegotiated deal or remain.
The party lost 59 seats in the general election last week.
Its MPs are embroiled in intense speculation about who will run to be leader, with Thornberry, the shadow Brexit secretary, Keir Starmer, and the shadow business secretary, Rebecca Long-Bailey, and backbench MPs Jess Phillips and Lisa Nandy all mooted as potential candidates.
Angela Rayner, the shadow education secretary, has said she is interested in the deputy position, vacated by Tom Watson who left parliament this election. Close Corbyn ally Richard Burgon is also said to want the deputy job.
The fallout from the election has led to a bitter war of words from MPs who lost their jobs angry at Corbyn’s team.
Jenny Chapman, who lost her Darlington seat, said she wanted Starmer to go for leader. He was instrumental in ensuring Labour supported another referendum.
She told BBC Radio 4’s Today programme on Tuesday morning that voters want “a leader they feel could be prime minister” and at the moment he is “the best proposition”.
Chapman said it was not essential for the next leader to be a women or from the north of England to win over voters in former safe seats. “Nobody on doorsteps of Darlington said the next leader has to have ovaries or a northern accent,” she added.
Thornberry has threatened to sue her former colleague Caroline Flint, who represented Don Valley, for “making up shit” about her.
The Islington South MP said she had consulted lawyers after Flint claimed on live television that Thornberry had once said to a fellow MP in a leave-voting seat that she was “glad my constituents aren’t as stupid as yours”.
Thornberry told Sky News: “I’ve contacted her and I’ve said to her: please withdraw, I’ll give you until the end of the day. And she hasn’t. So I’ve had to go to solicitors.
“People can slag me off as long as it’s true, I can take it on the chin. But they can’t make up shit about me – and if they do, I have to take it to the courts.”
