Enrollment in Medigap plans increased by 300,000 in the last year, from 9.9 million in December 2011 to 10.2 million in December 2012, according to new data from the health insurance industry.

America's Health Insurance Plans also said the fastest-growing Medigap plans include beneficiary cost-sharing.

AHIP touted its new findings that Medigap coverage is a "vital lifeline to more than 10 million Medicare beneficiaries" as it urged policymakers to avoid cutting benefits or enacting changes that "would harm the millions of seniors and people with disabilities who rely on Medigap coverage."