Cocaine may have been snorted in toilets just yards from the private hospital ward where the Duchess of Cambridge is due to give birth within the next few days.
Tests found traces of the drug in a cistern at St Mary's in London, according to a report in the Mirror.
The Mirror said it used drug testing wipes on a surface behind a toilet cistern in the Cambridge Wing, yards from the Lindo Wing maternity ward at the hospital.
The company behind the equipment told the Mirror that the tests "presumptively identify cocaine from trace amounts of residue present on surfaces that have been in contact with cocaine or handled with contaminated hands".
The paper says that hospital bosses have launched an investigation and will take immediate action against anyone abusing drugs at the hospital.
A spokeswoman for Imperial College Healthcare NHS Trust told the paper: "We take the safety of our patients and staff extremely seriously.
"We have 24/7 security cover at all our hospitals with regular patrols and CCTV covering key areas both outside and inside buildings, constantly monitored by our security teams.
"We work closely with police to investigate reports thoroughly and take action to ensure the safety of our colleagues, patients and their families.
"If there are signs that a person is abusing substances on one of our sites we will take immediate action to address this."