Emily Thornberry warned privately in September that Labour’s election chances would be hampered by taking a neutral position on Brexit.
Speaking at the party’s conference, for a BBC film being broadcast on Tuesday, she said she was worried about Jeremy Corbyn saying he “didn’t have a view” on the biggest decision facing the UK.
She was “really pushing” at the time for Labour to openly back Remain.
Labour’s defeat has led to a bitter internal row over its Brexit policy.
Some Labour candidates who lost their seats have blamed the party’s offer of another referendum for their defeat alongside doubts about Jeremy Corbyn’s leadership.
Ms Thornberry, the re-elected MP for Islington South who is expected to be a candidate in the contest to succeed Mr Corbyn, revealed on Monday that she had begun legal action against a former colleague who claimed the shadow foreign secretary called some Leave voters “stupid”.
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She said Caroline Flint’s claim she had told an MP from a Leave-voting area “I am glad my constituents aren’t as stupid as yours” was “a complete lie”. But Ms Flint, who lost her seat at the election, has stood by her remarks.
Labour went into the election offering another Brexit referendum on a new withdrawal deal it hoped to negotiate if it won power.
At its conference in Brighton, the leadership saw off an attempt by party members to force it to campaign to remain in the EU.
During the campaign, Mr Corbyn went further by saying that he personally would not take sides in any future public vote, arguing this would make it easier for him to implement whatever choice the people made.
While Ms Thornberry has never hidden her view that she thinks Brexit is a mistake, an interview she gave to the BBC’s political editor Laura Kuenssberg during the conference makes clear the extent of her doubts.
In the documentary, entitled The Brexit Storm Continues, she warned that a neutral position on Brexit would be politically dangerous.
She also revealed she had privately urged the leadership to take a much more overt pro-Remain stance.
“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, she said: “Well, I think it makes it more difficult and that’s why I’m really pushing this because I want Jeremy in Number 10.”
Greater Manchester Mayor Andy Burnham said the Labour leadership’s position on Brexit seemed to thwart the views of the party’s traditional supporters.
He told BBC Radio 4’s Today programme the Labour Party had always been a coalition between supporters in working-class communities and “university educated liberal left” and Labour “had not been speaking to both sides of that coalition for some time”.
Before he became mayor, Mr Burnham was the MP for the Labour stronghold Leigh, which elected a Tory MP last week.
It would “help” if the next Labour leader was from the North, Mr Burnham added, and he said he would lend his support to a candidate that supported devolution.
However, Labour’s Jenny Chapman who lost her Darlington seat in the election said it was “patronising” to think that “presenting someone who speaks with a northern accent means you are going to win support in the North”.
“I don’t think you need a particular accent to have empathy and compassion,” she said explaining she wants shadow Brexit secretary Sir Keir Starmer to run in the Labour leadership contest.
Mr Corbyn and shadow chancellor John McDonnell have apologised for Labour’s “catastrophic” performance, which saw them lose 59 seats.
The Labour leader said he was “sorry that we came up short”, while Mr McDonnell told the BBC: “I own this disaster.”
The Brexit Storm Continues was broadcast on BBC2 on 17 December at 21:00 and is available on the BBC iPlayer.