Yahoo said Wednesday personal data from over a billion users was stolen in a hack dating back to 2013 — twice as big as another breach disclosed just three months ago.

In a huge blow to the struggling internet pioneer, Yahoo said it made the discovery as it was investigating what was already the largest data breach of a single company.

“Yahoo believes an unauthorized third party, in August 2013, stole data associated with more than one billion user accounts,” it said in a statement.

Yahoo said this case “is likely distinct from the incident the company disclosed on September 22, 2016” affecting 500 million users.

The news poses a fresh threat to Yahoo’s deal to sell its core operating assets to Verizon for $4.8 billion.

In November, Yahoo disclosed that as part of its investigation into the prior breach, it had received data files from law enforcement “that a third party claimed was Yahoo user data.”