(Adds that Unum stopped offering new policies)

By Suzanne Barlyn

Jan 12 (Reuters) - Florida's insurance commissioner is allowing MetLife Inc and Unum Group boost long-term care premiums, alleviating financial stress the insurers have been facing from rising medical costs and policyholders living longer.

MetLife's Florida subdivisions can increase average monthly premiums by $4 to $44 over the next three years, while two Unum units can raise them by $5 to $55, Insurance Commissioner David Altmaier said on Thursday.

Increases will be phased in during an initial three-year period, beginning this year. Rates will be guaranteed for an additional seven years with no further increases, Altmaier's office said.

The agreements come at a difficult time for the long-term care insurance industry. Insurers must make good on policies written during the 1990s and later, when they significantly underestimated projected health-care costs and life spans.

"MetLife determined that a rate change on certain long-term care insurance policies was necessary due to cumulative changes in actuarial assumptions since the time these policies were initially priced," MetLife spokeswoman Kim Friedman said.

A Unum spokeswoman declined to comment.

Insurers have been pushing for U.S. state regulators to grant rate increases for long-term care policies. Last April, the Pennsylvania Insurance Department approved rate increases of between 20 and 30 percent for a total of 46,525 policies in the state.

Long-term care insurance covers expenses for nursing home or home care if the policyholder becomes incapacitated. Most of those expenses are not covered by Medicare, the U.S. government insurance program for the elderly and disabled, and can be extremely expensive out of pocket.

The median annual cost of a private U.S. nursing home room last year was $92,376, according to Genworth Financial Inc .

People over age 60 make up nearly 23 percent of Florida's 19 million residents, according to the Areawide Council on Aging of Broward County Inc.

Still, the impact of Florida increase and long-term care market overall is small for MetLife, given the company's diversified business lines, said Sandler O'Neill analyst John Barnidge.

Unum may feel the impact slightly more, given its position as a leading provider of disability insurance products, Barnidge said. But Unum does not have a large business in the long-term care market, Barnidge said.

Unum said it stopped offering new individual long-term care policies in 2008 and group policies in 2012.

(Reporting by Suzanne Barlyn; Editing by Lauren Tara LaCapra and Leslie Adler)