Campaigning ahead of the European Union referendum will remain on pause as politicians on both sides of the divide struggle to come to terms with the murder of Jo Cox.

Both of the main campaigns suspended their activities as news of the Labour MP’s death broke on Thursday and a series of engagements planned for today have been cancelled.

With days to go until the June 23 referendum, Leave campaign standard bearer Boris Johnson has insisted the Brexit backers are the underdog in the contest despite opinion polls swinging in their favour.

The prominent Vote Leave campaigner said a natural bias among voters towards the current state of things during big referendums means those backing Britain’s exit from the EU remain outsiders for victory.

But the Conservative MP claimed there is momentum in his campaign, with people willing to listen to the case.

Several polls in recent days have suggested Leave is ahead in the contest, but a high number of undecided voters makes it difficult for those seeking to accurately predict the outcome.

Mr Johnson, speaking before Vote Leave suspended its battle bus campaigning in East Anglia once it was known Mrs Cox had been attacked, was asked about the Brexit camp’s chances of victory.

He told reporters: “I’ve always thought it was very, very tight. I think we’ve been the underdogs for a long time and that must remain the case.

“In any big referendum like this there must always be an intrinsic status quo bias, but I think that things have certainly been moving our way and people are certainly listening to the case that we’re making.”

The former London mayor became the latest high-profile Tory to criticise Chancellor George Osborne’s threat of an emergency Budget in the event of Brexit and vowed to oppose any such measures.

He said: “It’s like saying would I support the tooth fairy being given a DBE (damehood) – it’s a fantastical proposition.

“If it were to appear I would certainly not support it, I would certainly oppose it, but I don’t think it will.

“I think what we’re seeing is the last fusillade of Project Fear.”