Boris Johnson’s adviser Dominic Cummings has told Brexit supporters that the general election is “much tighter” than polls might suggest and urged them to persuade their friends to vote Tory.

Cummings has worked closely alongside Johnson from the moment he entered No 10, though he has taken a back seat during the election campaign. With little more than a fortnight to go until polling day, he used his blog to send up what he called a “bat signal” to Vote Leave supporters, warning them Brexit is at risk.

“You will see many polls in the coming days. Some will say Boris will win. 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,” he said. “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.”

While the Conservatives’ lead over Labour has narrowed slightly in some polls in recent days, most still point to Johnson securing a decent majority, though many elections experts warn that the Brexit issue, which cuts across party lines, makes this a particularly difficult election to predict.

The perception that Johnson is sailing towards victory is also dangerous for the Tories, who will fear it may persuade their supporters to stay at home or embolden swing voters who might otherwise be too nervous of a Corbyn-led government to vote Labour.

Cummings lavished praise on Johnson, saying: “I saw him in meeting after meeting. He threw everything he had at it. The forces against us were very powerful. Most of the powerful people in Westminster supported remain.”

Corbyn channels Chumbawamba as he gets up again | John Crace Read more

He differentiated Johnson’s government from his immediate predecessor’s, blaming Theresa May and the former chancellor Philip Hammond for failing to channel resources to the NHS immediately after the Brexit vote.

He claimed that if Labour entered government, with the backing of the SNP, it would seek to “rig” a second referendum by allowing EU citizens resident in the UK to vote and giving voters the choice of “remain or remain”.

Corbyn has promised that a Labour government will negotiate what the party calls a “credible” leave deal, involving a closer relationship with the EU, within three months of coming to power and put that to the public in a referendum within six months.

Cummings claimed: “Apart from giving millions of foreign citizens the vote, they will rig the question so the ‘choice’ is effectively ‘remain or remain’, they will cheat the rules, they will do anything, supported by the likes of Goldman Sachs writing the cheques like they did in 2016, to ensure remain win.”

Cummings, well known for his obsession with data science, nevertheless said the most important thing Brexit supporters could do was talk to people.

“The most useful thing most people can do is make the time to speak to friends and family and explain why you will vote for Boris and why you think any other move means a Corbyn-Sturgeon alliance controlling Downing Street, which would be a disaster,” he said.

Labour is dramatically outgunned by the Tories in terms of funding, but has hailed its “people-powered movement”, and the Corbyn-supporting campaign group Momentum has trained thousands of activists in canvassing.

Cummings suggested Vote Leave supporters should tell their friends that if Johnson got his Brexit deal through parliament by 31 January, “all that babble in parliament with the MPs spinning their wheels will largely vanish from the news”.

He said they should say, as Johnson has done repeatedly, that Corbyn would waste the next two years on referendums, and added: “Respecting democracy is even more important than leave or remain. A cheated second referendum will be a disaster.”

It emerged after the publication of the post that Cummings had resigned as a special adviser to the prime minister. A Cabinet Office source said he stood down “following the dissolution of parliament” on 6 November.

While such a move is common among special advisers at election time and is understood to be a formality in Cummings’ case, it had been reported that he intended to stay on, with the Tory election campaign being run by Isaac Levido. It is unclear what role, if any, Cummings will play if Johnson is returned to Downing Street.