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, who is expected to be a candidate in the contest to succeed Mr Corbyn, revealed on Monday that she had begun legal