The BBC’s political editor, Laura Kuenssberg, has reignited the row over whether the Queen was in favour of Brexit, saying a source told her before the referendum that the monarch made comments supportive of leaving the EU.

Nine months after the Sun sparked controversy by publishing a headline, “Queen backs Brexit”, in March, Kuenssberg recalled what a contact had told her.

The front page caused one of the biggest rows of the EU referendum campaign and prompted a successful complaint to the press regulator, Ipso, by Buckingham Palace, which said it was misleading.

Speaking to BBC Radio 4’s Today programme, Kuenssberg revealed she had been told something similar but decided not to report it because it came from a single source. The BBC generally requires a story to be double-sourced before it can run.

“In a casual chat with one of my contacts, they said, ‘Do you know what? At some point this is going to come out, and I’m telling you now and I don’t know if the BBC would touch it, but the Queen told people at a private lunch that she thinks that we should leave the EU,’” she said.

“Apparently at this lunch she said: ‘I don’t see why we can’t just get out. What’s the problem?’

“My jaw hit the floor. Very sadly, I only had one source. I spent the next few days trying to prove it. I couldn’t find the evidence. Lo and behold, a couple of months later, someone else did. Of course then ensued a huge row between that newspaper and the palace over what had really been said or not said.

“There were lots of moments in the referendum campaign, but for me that was one when my jaw did hit the floor. Very frustratingly, the story did eventually emerge, whether it was true or not.”

The Sun stood by its story, saying it had two sources for the claim that the Queen had “let rip” at the then deputy prime minister, Nick Clegg, about Europe at a lunch at Windsor Castle.

Clegg said he could not recall the conversation and dismissed the Sun’s account as “nonsense”. He went on to point the finger at Michael Gove, the former justice secretary and leave campaigner, who had also been present.

Gove has never confirmed being the original source for the story, saying: “Well, as I’ve said before I don’t know how the Sun got all its information, and I don’t think it’s really worth my adding anything to what’s already been said about this story.”