Ministers and Whitehall officials have been warned not to try to hide sensitive government information by using private emails and text messages.
Information Commissioner Christopher Graham says it should “come as no surprise” that such are open to requests for disclosure.
His guidance follows reports private emails were used to conduct Department for Education(DfE) business.
The DfE denied the reports, insisting the emails were about Tory business.
The Commissioner has carried out a probe of Education Secretary Michael Gove’s department’s use of private email and will publish a ruling on the allegations early next year.
In his guidance published on Thursday Mr Graham makes clear that the Freedom of Information Act has always applied to all information – including emails and text messages – used by ministers and Whitehall officials for government business.
“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,” he says.
“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.”
The BBC’s Freedom of Information specialist Martin Rosenbaum said the commissioner’s intervention stems from reports in the Financial Times in September about the use of private e-mail by Michael Gove and his advisers.
Those reports were followed by further ones of civil servants using text messaging with the aim of evading FOI, and reports of “panic” in Whitehall at the prospect of such tactics being stopped.