Boris Johnson has accused Labour of trying to 'rig' a second Brexit referendum by allowing EU citizens to vote.
He has written to Jeremy Corbyn claiming that the move would 'alienate' voters.
This morning Michael Gove doubled down on denying EU citizens the right to vote, and said Tories need to take the prospect of Jeremy Corbyn becoming Prime Minister 'very seriously'.
Speaking on BBC Radio 4's Today Programme Mr Gove said allowing EU citizens to vote could sway a second referendum.
The policy was adopted by Labour - which says it supports a confirmatory vote on leaving the EU - in September.
Labour's Rebecca Long-Bailey said it is right to grant EU citizens the right to vote, as it affects their future.
Mr Gove said: "There's a basic principle of democratic fairness. It is the case that EU nationals don't vote, have never voted, in general elections, and therefore don't vote, have never voted in referendums like the Brexit referendum - and, therefore, we just think it would be unfair.
"It's a bit like a rugby league final, 13 players on either side. If one team suddenly said we're going to play rugby union instead and bring two extra players onto the field, that simply wouldn't be fair."
He added that he understands Labour's argument but that it is "fundamentally an assault on the basic democratic principle".
Ms Long-Bailey told the Today programme: “I think it’s fair to ensure that those people have the opportunity to vote over their future, because essentially this isn’t just about the future of people who were born here, it’s about the future of people who contribute to our economy and have done for a very, very long time.”
In a letter to Mr Corbyn, Mr Johnson claimed allowing EU nationals to vote would create "bitterness".
The Prime Minister wrote: "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.
"It would alienate millions who already feel disenfranchised and ignored by our political system."
He claimed that the Labour manifesto set out to "fiddle" a second referendum on Brexit.
