WASHINGTON — Just before Don Hall and his family left town for Thanksgiving, the laid-off manufacturing supervisor from Castalia, Ohio, wrote a $763.81 check to his health insurance company for his December payment.

He had paid $237 in November, but the big increase wasn’t due to rising health costs or a catastrophic illness — and it wasn’t an isolated incident.

Hall, 56, is among an estimated 7 million unemployed Americans who get a federal subsidy to help them buy health insurance under legislation known as the Consolidated Omnibus Budget Reconciliation Act.

For workers who are laid off or downsized between Sept. 1, 2008, and Dec. 31, 2009, the COBRA subsidy pays 65 percent of their job-based health insurance premiums for nine months.

That subsidy, however, expires Monday for Hall and untold thousands of others who began receiving it in March, when it first became available as part of the American Recovery and Reinvestment Act.

Unless Congress moves swiftly to extend the benefit, millions of other jobless Americans will experience the same sticker shock when they exhaust their subsidies and must pay full health insurance premiums, instead of just 35 percent.

With the subsidy, job-based coverage averages $398 a month for families and $144 for individuals, according to the Kaiser Family Foundation. Without it, premiums average $1,137 for a family and $410 for an individual.

In general, COBRA allows certain workers who lose their jobs — unless they were fired for gross misconduct — to continue their health insurance with their former employers for up to 18 months. Before the subsidy was offered, only about 9 percent of people who were eligible for coverage under COBRA took advantage of it because it was so expensive.

An August analysis by Hewitt Associates found COBRA enrollment had doubled with the subsidy.

It remains unclear when or whether Congress will address the subsidy expiration with specific legislation or as part of a major jobs bill.