The BBC reporter who sent a tweet mistakenly claiming the Queen had died committed "a serious breach" of guidelines, the BBC Trust has said.
The report said Ahmen Khawaja, a BBC Urdu reporter, had not been told the corporation was running a rehearsal of how it might handle a royal death.
The trust said the tweets - which were picked up by news outlets worldwide - were "a grave error of judgement".
Ms Khawaja is facing disciplinary proceedings over the tweets.
They were coincidentally published on the day the Queen was in hospital for her annual medical check-up.
After speculation about her well-being was raised on social media, Buckingham Palace took the unusual step of confirming the routine appointment.
The BBC Trust, which is the corporation's governing body, said "some news staff" had been sent an email informing them of the royal obituary rehearsal and telling them not to discuss it on social media or elsewhere.
However, it continued: "A journalist working for the BBC's language services, who had not been sent the email, saw an internal TV monitor which was showing the rehearsal.
"A number of tweets were sent from her Twitter account. The first stated that the Queen was being treated in hospital, the second stated that the Queen had died; the tweets included a link to BBC World's official Twitter feed."
The Trust said the tweets were "a grave error of judgement" and it "profoundly regretted any distress that had been caused".
It added that while the tweets had not come from an official BBC Twitter account, the links to the BBC World account - and the journalist's position with BBC Urdu - would have caused greater confusion and disseminated the mistake more widely.
The BBC executive, which manages the corporation, agreed that it viewed the incident as "a serious breach of the editorial guidelines", and said an internal inquiry into what happened was continuing.