"If he wasn’t referring to that he must have been not listening. I think he is a reluctant remainer.”

During a visit to Aberdeen on Saturday, Mr Corbyn said that while the EU is not "perfect", the "employment rights" which are enshrined in European law "are very important".

A vote to stay in the UK is "not an endorsement of every bureaucratic excess of the European Union", Mr Corbyn said, claiming what is important is "a principle about working with people across national borders".

A poll by ORB for The Independent newspaper on Saturday put the Leave campaign at 55 per cent and support for Remain on 45 per cent, after allowing for an individual's likelihood to vote.

One Labour source said party workers and MPs felt “very emotional” about Britain leaving the EU. “We already knew we were in the fight of our lives and that it was going to be incredibly close," the source said.

“When that poll was announced there were a lot of people in tears in the office. A lot of us are very emotional about this – more so than with a general election because this will be a one-off vote. We are only going to have one chance to get this right.”