Washington ? Millions of seniors face double-digit hikes in their Medicare prescription premiums next year unless they shop for cheaper coverage.

A new analysis of government data finds that premiums will go up an average of 10 percent among the top plans that have signed up some 70 percent of seniors. That’s according to Avalere Health, a private research firm that crunched the numbers.

Marketing for next year’s drug plans gets under way Oct. 1, and seniors will see some of the biggest changes since the Medicare prescription benefit became available in 2006. More than 17 million are enrolled in private drug plans offered through Medicare.

“People are just going to have to get on top of this and shop around,” said Dan Mendelson, president of Avalere, which does research for industry and government. “Beneficiaries are really going to have to reassess their plans for next year.”