Millions of U.S. senior citizens face double-digit premium increases for major Medicare plans next year as major insurers raise fees, according to a new study by research firm Avalere Health.

Seven of the 10 top-selling Medicare programs are raising fees between 11 percent and 23 percent, according to analysis from Avalere.

Humana Inc. (NYSE: HUM) and Wal-Mart Stores Inc. (NYSE: WMT) plan to increase premiums of its Preferred Rx Plan by 23 percent to $18.50 per month. Coventry Health Care Inc.'s (NYSE: CVH) First Health Part D Premier and Value Plus Plans are set to rise 17 percent and 18 percent, respectively. Cigna Corp. (NYSE: CI)'s Medicare Rx Plan premiums expected to increase by 15 percent.

Avalere said that consumers can avoid the fees by switching to new, lower-cost plans.

"Seniors need to carefully assess their options going into this open season to ensure that they have a plan that meets their needs," said Bonnie Washington, senior vice president of Avalere Health, in a statement.

UnitedHealth Group Inc.'s (NYSE:UNH) Medicare Rx Save Plus PDP has fees of $15 per month, one of the lowest in most markets. First Health, Conventry and CVS Caremark also have premiums below $30.

The average premium for Medicare plans will remain unchanged at $30 per month in 2013, a number that Obama officials announced in August.

Shares of Humana were down 67 cents to $69.84 in Wednesday morning trading. Wal-Mart gained 12 cents to $74.38. Cigna was up 18 cents to $47.03. Coventry fell 17 cents to $41.63. United was up seven cents to $56.27.