It was the WhatsApp message that convinced millions of people the Queen had died. Now, the mystery of how a man called Gibbo and his friends inadvertently created a fake news panic that went “bigger than Ben Hur” has apparently been solved – and the finger of blame is pointing at a military drill near Yeovil.

On Sunday night, millions of Britons received messages suggesting the monarch had passed away, often forwarded on WhatsApp, Facebook and Twitter, prompting Google searches for the Queen to spike and forcing Buckingham Palace to insist the 93-year-old was very much still alive.

The main source for the claim was a screengrab of a mysterious WhatsApp group where a man identified only as Gibbo had posted a message claiming the Queen’s reign had ended. In Gibbo’s telling, she had had a heart attack on Sunday morning, but the news was being kept under embargo until Monday to allow the military time to prepare.

“Queens passed away this morning, heart attack, being announced 930 Am tomorrow, channel dash 0800, tomorrow in full number 1s, in your black kit bag you need: 1 set of 3s, 1 set of 4s, underwear and socks for 2 weeks, washing kit, body washing kit, cities fornstand down,” said the message, forwarded by Gibbo from an unidentified original source.

Other members of the WhatsApp group named Old Times – identified as Burnsy, Cheeks, Morty, Ricey and Josh – expressed doubts. But one individual took the fateful decision to screengrab their private conversation and share it outside the group, setting off a chain of events that created a fake news crisis.

A screengrab of the WhatsApp message that helped spread false rumours the Queen had died. Photograph: Burnsy, Cheeks, Gibbo, Josh, Morty, Ricey/WhatsApp

Despite signs the WhatsApp screengrab may not be a reliable news source – the group’s photo was of an unidentified penis – the British public lapped up the supposed insider knowledge, sending it viral on social media.

The story could have ended when the Queen was confirmed to be alive and a palace source said it was “business as usual”, were it not for the intrepid digging of the journalist Tom Cotterill from the Portsmouth News.

Following a tip from a source, he confirmed there had been a kernel of truth in the original message. Military staff at RNAS Yeovilton – a joint navy and army base in Somerset – had indeed been taking part in a drill. The individual said it was part of recall procedures for London Bridge, the codename for the massive operation that will swing into place when the monarch eventually dies.

The extensive planning for London Bridge has been in place for decades, with the news of the Queen’s death to be transmitted via secure lines to the prime minister and other key officials before being announced through official news channels. When the Guardian published an in-depth account of the plans in 2017, there was mention of 41-gun salutes, parliament being recalled and thousands of military personnel being deployed ahead of the Queen lying in state in Westminster Hall. Despite substantial planning on how to handle media preparations, there was no mention of WhatsApp.

Nonetheless, it appears an individual who received the practice recall message had believed it to be real and forwarded it to a friend. It eventually ended up on Gibbo’s phone and ultimately on the screens of millions of people around the world.

So the Queen is NOT dead. Wow, even ‘Gibbo’ is lying to us now | Joel Golby Read more

A key distributor of the fake WhatsApp screengrab was the former paratrooper Alfie Usher, who shared it to a military humour Facebook page called Fill Your Boots UK, insisting he felt it was an obvious joke.

“It went bigger than Ben Hur,” he told the Portsmouth News. “I realised I was losing control of the situation when other news sources were picking it up. I feel so sorry for Gibbo. Someone screenshotted his message, which was out of his control, and he will be the one that gets shafted.”

A Royal Navy spokesperson said there had been a drill at the base, which is home to many of the UK military’s helicopter operations: “We can confirm an internal exercise took place at Royal Naval Air Station Yeovilton in line with established contingency plans for recall of personnel. These exercises are conducted on a regular basis and no significance should be drawn from the timing of the exercise.

“While the exercise was conducted properly, we regret any misunderstanding this may have caused.”

Cotterill said he had been amazed to see the story spread around the world but was glad to have uncovered the original source of the misunderstanding: “This is one of those bizarre conversations where screenshotting a military conversation has gone out of control and created a worldwide issue.”

The identity of Gibbo – and whether he is a serving member of the British military – remains unknown. There is no indication he is in any rush to go public and be known as the man who inadvertently announced the death of the Queen.