The prime minister’s claim sparked fury among representatives of EU nationals living in the UK, who said it appeared to be part of an attempt to “demonise” them in the eyes of their British-born neighbours.
And Labour retorted that if the PM had any confidence in his Brexit deal, he would be willing to put it back to the people for a confirmatory vote.
Conservatives released figures suggesting that Leave would struggle to win a second EU referendum if all adult residents of the UK were permitted to vote.
More than 2 million adults with EU passports living in the UK were barred from voting in the 2016 referendum, when their votes could have swung the result for Remain, which lost by a margin of less than 1.3 million of the 33.6 million votes cast.
Labour’s manifesto pledges to grant voting rights to all UK residents, but does not make clear whether this would take place before the Final Say referendum which Jeremy Corbyn has promised for June 2020 if he becomes prime minister.
In a letter to the Labour leader, Mr Johnson warned that allowing EU citizens to vote in a second referendum would create “the worst crisis in democratic politics in over a century” and urged him to reconsider the policy.
“Your manifesto sets out plans to fiddle your second referendum on Brexit,” wrote the prime minister. “You want to give two million EU nationals the vote in your referendum. This is a sly attempt to undermine the result of the 2016 referendum, and is profoundly undemocratic. No true democrat, even the most ardent supporter of Remain, could support your attempt to undermine the result of a democratically expressed vote.”
Mr Johnson said that allowing a referendum vote to EU national residents would create “incredible bitterness that might take decades to repair”.
“Imagine how people will feel if the biggest democratic exercise in our history is overturned because you gave two million EU citizens the power to reverse Brexit,” he said. “It would alienate millions who already feel disenfranchised and ignored by our political system.”
But Maike Bohn, of the 3MIllion group representing EU nationals in the UK, said that extending the vote to permanent residents of continental origin would be “rectifying a defect of the first referendum, as those most affected by Brexit did not have a say”.
Ms Bohn told The Independent: “I don’t like the use of words like ‘fiddling’ and ‘rigging’ in this context. For me, it taps into this really alarming demonisation.
“The government tells you ‘You are our friends, we want you to stay’, but then they say that we are draining the NHS and the welfare system and they have to have more security at the borders. It is irresponsible and it is toxic.
“EU citizens make a net contribution to this country. We pay our taxes and we have every right to have a say in our destiny.”
According to Conservative calculations, the Leave side would need an additional 483,344 votes on top of the 17.4 million it secured in 2016 to win a referendum in which EU national residents had a vote.
With opinion polls over the past two years consistently showing a majority of the electorate now wish to Remain in the EU, this might prove an impossible hurdle to overcome in a second referendum.
Labour’s shadow minister for voter engagement, Cat Smith, said: “The Labour Party believes in democracy. We welcome securing a new and better Brexit deal and putting it back to the people to have the final say.
“If Boris Johnson had any confidence in his own deal, he would have let it be scrutinised by parliament and then put it back before the people for their final verdict.”