Google is facing an investigation by regulators over a secret project which gave it access to health data from millions of hospital patients.

The firm is under growing pressure over Project Nightingale, a partnership between Google and Ascension, which operates 150 hospitals. It is now the subject of an investigation by the US Department of Health amid concerns around the collection and use of sensitive health data.

Watchdogs will consider whether the partnership clashes with the Health Insurance Portability and Accountability Act, which protects the health information of patients.

The deal between Google and Ascension – which bills itself as a non-profit Catholic health system – allegedly gives up to 150 Google employees access to data such as lab results, doctor diagnoses and hospitalisation records. It amounts to a complete health history for each patient, the Wall Street Journal reported.

Google has teamed up with Ascension to gather information across 21 states since last year in a move that underlines its ambitions to move into the healthcare industry.