Ministers and officials can be ordered to hand over emails sent from their private accounts if they contain details about Government businesses, a watchdog has confirmed.
New guidance sets out how non-governmental email addresses, as well as text messages, are covered by the Freedom of Information Act (FOI).
It follows concerns raised in September about the way requests for information were handled at Michael Gove's education department after claims official business was conducted using personal emails.
The Information Commissioner's Office (ICO) was asked to look into allegations that messages containing sensitive information were being kept away from the department's own civil servants as well as the public.
Information Commissioner Christopher Graham said on Thursday: "It should not come as a surprise to public authorities to have the clarification that information held in private email accounts can be subject to Freedom of Information law if it relates to official business.
"This has always been the case - the Act covers all recorded information in any form.
"It came to light in September that this is a somewhat misunderstood aspect of the law and that further clarification was needed.
"That's why we've issued new guidance today with two key aims - first, to give public authorities an authoritative steer on the factors that should be considered before deciding whether a search of private email accounts is necessary when responding to a request under the Act.
"Second, to set out the procedures that should generally be in place to respond to requests.
"Clearly, the need to search private email accounts should be a rare occurrence; therefore, we do not expect this advice to increase the burden on public authorities."