Compared to the Conservatives, Labour is remarkably united over the EU, with the entire shadow cabinet and 214 of its 231 MPs backing the party's In campaign. Only a handful have joined the rival Labour Leave group, though sources are confident that more, potentially including shadow ministers, will do so when David Cameron's renegotiation concludes.

But there are notable tensions within the In campaign. At this week's shadow cabinet meeting, which received a presentation from pro-EU head Alan Johnson, Andy Burnham warned of electoral damage to Labour if it was part of the "establishment case" for staying in. Burnham emphasised the need for the party to differentiate itself from Cameron and business leaders, I'm told. Angela Eagle also spoke of her concern at the number of eurosceptic Labour supporters.

Just as the SNP surged following the Scottish independence referendum, so some shadow cabinet members believe Ukip could do so after the EU vote. One told me of his fear that those Labour supporters who voted Out would make "the transition" to voting for Farage's party. Ukip finished second in 44 of Labour's seats at the last election and helped the Tories win marginals off the opposition.

Among Labour's pro-Europeans, the fear is that the party's campaign will be "half-hearted". Jeremy Corbyn, a long-standing eurosceptic (who some believe would have backed withdrawal had he not become leader), struggles to express enthusiasm for remaining In. Speaking to the New Statesman, former shadow Europe minister Emma Reynolds warned:

"The British public will expect the Labour Party to have a clear position. And we do have a clear position and that's that we're going to campaign to stay in the EU. Trying to fudge the issue or hedge your bets is not going to go down well with the British public. Of course we need to talk to people about all aspects of the EU, and that will involve talking to people about immigration, but there isn't a 'maybe' on the ballot paper it's a binary choice between remain and leave. We have to be clear with people where we are because they won't thank us for being wishy-washy."

Labour's Brexiters draw comfort from the dearth of MPs campaigning for EU membership. Kate Hoey told me: "I have been genuinely surprised how few supposedly 'pro-EU' Labour MPs have been prepared to come out and speak publicly of their support for staying in. They know, as those of us campaigning on the Leave side know, that thousands and thousands of Labour supporters, all over the country, want to come out and they are not going to receive a great reception on the doostep".