Documents on UK-US trade talks, leaked ahead of the 2019 general election, were stolen from an email account belonging to Conservative MP Liam Fox, it has emerged.
The papers were published online and used by Labour in the 2019 campaign to claim the NHS would be put at risk.
The UK government has said Russians almost certainly sought to interfere in the election through the documents.
A criminal inquiry into the leaking of the documents is underway.
A spokesman for the National Crime Agency confirmed it was leading the investigation, but added he could not comment further.
Mr Fox was international trade secretary from July 2016 to July 2019.
It is not clear when his account was accessed and the information stolen.
Reuters, which first reported the story, said hackers accessed Mr Fox’s account multiple times between 12 July and 21 October last year.
A government spokesperson said: “There is an ongoing criminal investigation into how the documents were acquired, and it would be inappropriate to comment further at this point.
“But as you would expect, the government has very robust systems in place to protect the IT systems of officials and staff.”
A spokesperson for the National Cyber Security Centre said it works closely with MPs and political parties to offer them “the best cyber security guidance and support.”
“We have worked closely with political parties for several years on how to protect and defend against cyber attacks – including publishing advice on our website.
“There is an ongoing criminal investigation and it would be inappropriate to comment further at this stage.”