Theresa May has held secret discussions over a three-way second referendum ahead of a crunch meeting with Labour this week to agree a cross-party Brexit deal.

The Prime Minister has carried out “scenario planning” with aides and ministers in case the Government cannot prevent a Parliamentary vote on a second referendum.

John McDonnell, the shadow Chancellor, said it “may well” be the case that any deal would have to be put to a second referendum, adding: “I think the Conservatives have to recognise that if a deal is going to go through there might be a large number of MPs who will want a public vote."

Mrs May and her advisers are understood to have 'war gamed' the possibility of giving voters a choice of her deal, no deal and remain, though Government sources insisted it would only become relevant if talks with Labour collapse and Parliament forces a vote on a second referendum.

That could happen either through a series of “indicative votes”, which are Mrs May’s plan B if the Labour talks fail, or through an amendment to Brexit legislation.