Boris Johnson’s top adviser Dominic Cummings has used a blog post to send up what he said was a “bat signal” to Brexiteers, warning them not to be complacent about the result of the general election.
With a projection of the result at the halfway stage in the campaign set to be published by pollsters YouGov, Mr Cummings warned the contest was “much tighter” than it appeared.
The Downing Street adviser said there was still a “very real possibility” of a hung parliament despite the Conservative Party maintaining a significant lead over Labour.
He said that voting for Nigel Farage's Brexit Party in key Conservative target seats in the Midlands and the North of England risked handing victory to Labour.
Writing on his blog, Mr Cummings - who is credited with masterminding the successful Vote Leave campaign in the 2016 referendum - said if there is no Tory majority, the other parties would combine to stop Britain leaving the EU.
He said Labour and the SNP would combine to "cheat" a second referendum by giving the vote to EU nationals in order to secure a Remain vote.
Cummings also appeared to undermine one of the central claims of the Tory campaign - that Labour and the SNP would hold two referendums next year, on Brexit and Scottish independence - by initially writing that indyref2 would take place in 2021.
The blog post was quickly edited to read: “And there’ll be another Scottish referendum.”
Mr Cummings is not thought to be playing a direct role in the Tory campaign, and was reported to be taking time out to undergo surgery.
As a special adviser in Downing Street, he would be expected to stand down from his post in order to take part in the election campaign.
“If Boris doesn’t get a majority, then Corbyn and Sturgeon will control the government,” he wrote.
“Their official policy is to give the vote to millions of foreign citizens to cheat their second referendum.”
Cummings went on: "You will see many polls in the coming days. Some will say Boris will win," he wrote.
"Trust me, as someone who has worked on lots of campaigns, things are MUCH tighter than they seem and there is a very real possibility of a hung parliament.
"Without a majority, the nightmare continues. ALL other MPs will gang together to stop Brexit and give EU citizens the vote. It's that simple."