UK Anti-Doping (Ukad) has confirmed it was the target of a cyber attack over the weekend.
Ukad holds drug test details and medical records of thousands of athletes, including Premier League footballers and Olympians.
It said no systems were compromised and no data was lost, adding: "We are satisfied we have appropriate levels of cyber security in place".
Ukad has not said if it knows who was behind the attack.
"We took the necessary steps to investigate and resolve the situation," a statement from the body said. "No core activity, including our testing programme, has been impacted."
No-one has yet claimed responsibility.
Russian-based hacking group Fancy Bears has previously stolen athletes' medical data from the World Anti-Doping Agency.
It began releasing athletes' data in 2016 and focused largely on naming those who had sought Therapeutic Use Exemptions (TUEs), which allow athletes to take banned substances for verified medical needs.