Average branded prescription drug prices have nearly doubled over the last five years, according to the nation’s largest pharmacy benefits organization.

In a report that could fuel the growing political outcry against high drug prices, pharmacy benefits manager Express Scripts said “opportunistic manufacturers” and “scheming” pharmacies helped drive up prices by 98.2 percent since 2011.

High prescription drug costs have become an issue in the presidential primary campaign, with candidates in both the Democratic and Republican races proposing measures to control prices. And “Pharma bro” Martin Shkreli became the face of the issue in September when he jacked up the price of a lifesaving AIDS and cancer drug from $13.50 to $750 per pill.

Express Scripts, which negotiates contracts for drug coverage on behalf of insurers and employers, said in the report that price “increases are occurring with greater magnitude and frequency than in recent years” and that “in 2015, nearly one-third of branded drugs experienced annual price increases of 20 percent.”

It blamed consolidation in the prescription drug market and “price gouging” for “diluting the value of generic medications and lower-cost therapies.”

Branded drug prices increased by 16.2 percent in 2015, Express Scripts said. But the growth in overall spending on drugs slowed by using more generics.