He hit back at the former foreign secretary, who alleged that aides with “great influence” over Mr Corbyn were blocking a shift to outright support for a fresh referendum.
“The idea that Jeremy doesn't make his own decisions, or decide what he wants to do, is laughable,” he insisted.
“Jeremy has his own views and takes his own decisions and decisions are not taken for him by anybody.”
He said it would be mounted “at the time we judge it to have the greatest chance of success” – which is widely assumed to be after the summer recess.
Speaking on the BBC’s Today programme, the veteran Labour MP attacked aides who “don't give a toss” about the views of Labour members, adding: “They just are determined to make sure we don't do anything to impede Britain leaving, if necessary with no deal.
“I think there are people very close to him, with great influence with him, who are and have been from the beginning passionately opposed [to EU membership]”, Dame Margaret alleged.
Her target is believed to be “the four Ms” in Mr Corbyn’s team, all known opponents of a fresh Brexit referendum and some with a long antipathy to the EU.
They are Seumus Milne, his director of communications, Karie Murphy, his chief of staff, Andrew Murray, his political adviser, and Len McCluskey, the head of the powerful Unite trade union.
The spokesperson also insisted Mr Corbyn was striving to “find a common position” on a further referendum, which was expected to reach a conclusion “in the next few weeks”.
He added: “That's his way of leading, it is to try and bring people with him and find common ground and that's what he is doing here.”
Labour MPs demanding another referendum are desperate for Mr Corbyn to start campaigning for it – and to guarantee the party would back Remain if it took place.
They were alarmed, last week, when the leader said any referendum ballot paper should contain “real choices for both Leave and Remain voters”, leaving open the question of which he would back.
The spokesperson owned up to shadow cabinet splits, with some members urging “commitments about what should be on the ballot paper”.
Labour “expects” Remain to be one option, he said, but said “parliament must decide” the alternative, saying Labour’s softer Brexit proposals were still “on the table”.
The comments will alarm Final Say supporters, implying Mr Corbyn is still some distance from full-throated backing for Remain.
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