Several hundred retired Massachusetts teachers could see massive health insurance price spikes next year, if lawmakers do not act.

Currently, up to 955 retired teachers who are insured through the state's Group Insurance Commission will have to switch plans and could see premium increases of 20 percent to 80 percent.

Gov. Charlie Baker on Friday introduced a bill that could mitigate the price spikes, and the House and Senate are likely to take it up at an unusually fast pace.

"We'll try to get it done as soon as possible," said Rep. Jerald Parisella, D-Beverly, chairman of the Joint Committee on Public Service.

The Group Insurance Commission was recently forced to backtrack on planned changes to public employees' health insurance after a public backlash to a proposal that would drop three of its six insurance carriers.

Now, after some changes to plan design and pricing, of the GIC's 440,000 members, around 84 percent will see their premiums either increase by less than 2 percent or decrease.

But the GIC also covers approximately 10,000 retirees, mostly teachers, in a separate pool, referred to as "Pool 2." They are a generally older population, which means they tend to have higher health care costs. A 1972 state law requires them to be covered separately. This group of retirees comes from 46 towns and school districts, with the last ones having joined in 1996. The separate pool was established before municipalities began to be allowed to join the GIC in 2007.

This year, only one health insurance company -UniCare - bid to cover that population. UniCare already covers the majority of these individuals. All of the other plans have dropped out, including Harvard Pilgrim, Fallon, Tufts, Health New England, and Neighborhood Health Plan.

"The evolving dynamics of our health insurance marketplace have created a situation in which having a relatively small group of retirees pooled separately from the rest of our members is no longer financially viable," said Roberta Herman, executive director of the Group Insurance Commission.

Retirees and their families in this pool who are today insured by the Springfield-based Health New England would see some of the biggest price spikes, an increase of around 74 to 80 percent.

Baker's bill would let the retirees join the general GIC pool. Baker wrote in a filing letter that because there are only 10,000 retirees, adding them to the GIC pool would not materially affect premiums. Adding this group is estimated to cost other members between 0 and .03 percent, Herman said. The move would let most of these Pool 2 retirees keep their current plans, avoiding price spikes.

Going forward, municipalities would no longer be able to join this risk pool, which none have done anyway for more than 20 years.

Lisa Adams of the Massachusetts Municipal Association said Tuesday, at a public hearing before the Joint Committee on Public Service, that municipalities support the change as a way to lower health care costs. Communities that pay part of the cost for retirees' health insurance will be hit with additional costs if the insurance premiums go up.

Shawn Duhamel, legislative director of the Massachusetts Retirees Association, said "there is no downside" to passing the bill, from the perspective of improving choice and affordability for these retirees.

Sean King, of the Massachusetts Teachers Association, said the move would provide "much needed financial relief and choice" to the retired teachers.

Open enrollment begins April 4, 2018 for the GIC, so lawmakers must act quickly if they want to alleviate the premium increases. The GIC has delayed communicating with these members about open enrollment until GIC officials know whether lawmakers intend to act.

Parisella said he expects the committee to vote to recommend the bill on Tuesday. Leaders in both the House and the Senate appear supportive. But the timeline for acting on the bill will depend on when lawmakers hold formal sessions, which could get postponed due to this week's impending snowstorm.

"The testimony today is a good sign that it seems like all interests and parties are supportive of it," Parisella said. "We'll try to poll it out today and try to expedite this just because we know how much of a financial impact this could have on retirees."