Chris Bryant, the recently resigned Shadow Cabinet Minister, has claimed Labour leader Jeremy Corbyn may have voted Leave in the referendum.
Mr Bryant was the Shadow Leader of the House until joining the mass resignations from Mr Corbyn's cabinet following the Brexit vote.
Asked about Mr Bryant's claim, a spokesman for Mr Corbyn wrote in a text to The Independent: "That's nonsense. These are simply briefings aimed at destabilising the leadership."
In an interview with BBC News, Mr Bryant said Mr Corbyn had refused to tell him how he had voted in Thursday's referendum.
"His attitude in the EU referendum was so casual, so, I think, inappropriately inept and sometimes it felt as if nobody knew if he was in favour of Remain or Leave," Mr Bryant said.
"People on polling day itself were saying to me, 'how's he voted, has he voted to leave?'
"I suspect that Jeremy may have voted to leave."
As he explained why he had decided to resign from the Shadow Cabinet, Mr Bryant added: "Not only is that [a Leave vote] a betrayal of Labour's historic position on the European Union – a fundamental economic and foreign policy objective of ours – but also it means that if he were to lead us into a general election, the latest poll shows we would lose 150 seats, we would be a rump of 75 Members of Parliament."
Asked if he could state categorically that Mr Corbyn voted Remain, the Labour leader's spokesman said: "Of course. But it's unacceptable to ask him how he voted.
"He led the Labour In campaign and we were the only party to deliver a majority of our supporters."
