Europe's drugs regulator has started an infringement procedure against Swiss pharmaceutical group Roche Holding to investigate its alleged failure to report fully tens of thousands of cases of possible adverse drug reactions.

If found guilty, the world's biggest maker of cancer medicines could be fined up to 5% of its annual turnover in the European Union.

It is first time that the London-based European Medicines Agency (EMA) has launched such a procedure since legislation covering the action came into force five years ago.

"The agency will report the outcome of its investigation to the European Commission who may impose fines or periodic penalty payments ... if it finds that Roche has committed an infringement of its obligations," the EMA said on Tuesday.

The issue of lax drug-safety reporting by Roche was first highlighted by the agency in June after a routine inspection by British investigators found it had failed to assess properly up to 80,000 cases of possible adverse drug reactions.

The issue relates to medicines from across the Roche product range that were part of a financial reimbursement system in the United States.

Roche said it was cooperating with health authorities, adding that, to date, no impact on the safety profiles of any of its products had been found.

The EMA also said all Roche's medicines remained authorised without changes to the treatment advice for patients and doctors, since there was no indication of "a negative impact on the benefit-risk balance" of the drugs.

The agency's investigation affects 19 Roche drugs that have been centrally authorised - including blockbuster cancer treatments like Herceptin and Avastin and its popular flu drug Tamiflu - as well as some medicines that were approved by national authorities.