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An inquiry is under way after official correspondence about procedures for when the Queen dies was accidentally leaked by the Welsh government.
The correspondence and a security pamphlet marked "official sensitive", was emailed to a member of the public.
The leak included a letter from a top UK civil servant expressing concern that a news website had disclosed confidential details.
The Welsh government said that no operational information was revealed.
But it added that the pamphlet should not have been shared.
A spokesperson for the Welsh government's most senior civil servant, Dr Andrew Goodall, said it took the "issue of information and data security very seriously" and the incident was being investigated as a "potential security breach".
In emails seen by BBC Wales, in September 2021 a top UK government civil servant wrote to other officials expressing concern that a news website had published new details about the handling of the Queen's death.
The civil servant said the news report was based on internal documents which were not intended to be made public.
In response to this, another senior civil servant asked for "bespoke guidance" on security to be drawn up and shared within the governments of the UK.
This security guidance was marked "official sensitive", and was sent by email to a member of the public by a Welsh government official in error.
In his full statement, the Welsh government's Permanent Secretary, Dr Goodall said: "Whilst the email did not contain any operational information, it was marked 'official sensitive' and should not have been shared.
"We take the issue of information and data security very seriously and this is now being investigated as a potential security breach.
"We are unable to comment further."
Analysis by Mark Palmer, BBC Wales politics
Keeping sensitive information under wraps is a challenge for many organisations, whether they are public bodies or commercial entities.
Although checks and security systems are in place, leaks do happen - whether they are intentional, or in this case, as the Welsh government stresses, accidental.
The leaked pamphlet is marked "official sensitive" and offers general advice on "good security behaviours" and how best to go about "protecting information".
However, although the Welsh government says no operational material was revealed, it is highly embarrassing for them as it involves the Queen.
An investigation is underway and government officials can expect further guidance on what they can do to ensure that an error like this does not happen again.