Shadow foreign secretary Emily Thornberry was among Labour MPs who joined thousands of protesters marching through Brighton supporting a second referendum on Brexit.
Ms Thornberry and others could be heard chanting “What do we want? People’s Vote! When do we want it? Now!”.
But as ITV News Political Correspondent Paul Brand says, that is a direct contradiction to policy agreed at Labour's National Executive Committee.
Paul said an NEC statement released this morning "would mean referendum 6 months after an election".
It also flies in the face of what leader Jeremy Corbyn has been saying regarding the timing of a second EU referendum.
Mr Corbyn has repeatedly said he will only call for a new Brexit vote after winning a general election and negotiating his own deal with the EU.
And Labour Party chairman Ian Lavery agrees Mr Corbyn has the best policy in his aim of negotiating a deal before offering it to the people to vote upon.
He said Mr Corbyn's strategy means no one is left behind, Remainers or Leavers - but there are suggestions that sitting on the fence will be attractive no one.
Mr Lavery passionately defended the party against that suggestion, telling ITV News Political Correspondent Shehab Khan: "If you think walking down the middle is a fudge, I feel sorry for you.
"The reality is, we're saying to the Remain voters that 'we'll not leave you behind' and we're saying to the Leave voters 'we'll not leave you behind'."
He added: "What Jeremy Corbyn is trying to do is bring this country together because Brexit has torn the country apart."
Protesters heard speeches from Ms Thornberry, as well as Labour shadow Brexit secretary Sir Keir Starmer and other figures from the political left, including Green MP Caroline Lucas.
Sir Keir told the crowd a second referendum is the "only way out" of the current impasse, as he said he would support Remain.
Addressing the Trust the People rally in Brighton, he said: "I have to admit that a year or so ago I wasn't sure that a referendum was the right way out - but now I'm utterly convinced it's the only way out.
Ms Thornberry told the crowd at the People's Vote rally that Labour should lead the campaign to Remain in the EU if there is a second referendum.
She added: "We're not going to let it happen that we crash out of Europe without a deal.
"We must make sure that there is a second referendum and Remain is on the ballot paper and Labour campaigns for Remain - and not just that, Labour should lead the campaign."
Labour MPs, along with thousands of supporters are in the seaside city for the Labour Party conference.
A Labour activist - Jon Lansman, founder of campaign group Momentum - had put a motion to the National Executive Committee for a vote on whether to remove the post of deputy leader - currently held by Tom Watson.
But fears of Mr Watson being stripped of his role were put to bed for the time being, with Mr Corbyn cancelling the vote.