The Metropolitan police’s counter-terrorism command launched a criminal investigation on Friday, and on Sunday, the Sunday Times, citing unnamed government sources, reported that a suspect had been identified and the possibility of a computer hack by a foreign state ruled out.

“They think they know who did the leaking,” the paper quoted an unnamed government source as saying. “It’s now a case of building a case that will stand up in court. It was someone with access to historical files. They went in and grabbed a range of material. It was quite crude.”

Both the Sunday Times and the Mail on Sunday reported that intelligence officials from GCHQ were about to join the investigation, to attempt to find the suspect by examining email and phone records.