Media reported that De Piero wrote to party leader Jeremy Corbyn to say she did not believe he could deliver a Labour victory at a general election, which may take place within months after Conservative Prime Minister David Cameron announced on Friday he would step down by October.

Earlier, Labour's foreign policy chief Hilary Benn was sacked after saying he had lost confidence in Corbyn, and health policy chief Heidi Alexander resigned.

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Corbyn loyalist John McDonnell, the economy policy chief, urged the party to stay together and said Corbyn was "not going anywhere".