A new fight is brewing over health insurance companies letting millions of Americans renew their current coverage for another year — and thereby avoid changes under the federal healthcare law.

That may offer a short-term benefit for certain consumers and shield some of those individual policyholders from potentially steep rate increases. But critics say this maneuver could undermine government efforts to remake the insurance market next year and keep premiums affordable overall.

At issue is a little-known loophole in President Obama’s landmark legislation that enables health insurers to extend existing policies for nearly all of 2014. This runs contrary to the widespread belief that all health insurance must immediately comply with new federal rules starting Jan. 1, when most provisions of the law take effect.

“Insurers are onto this, and the big question is how many will try to game the system,” said Timothy Stoltzfus Jost, a law professor and health policy expert at Washington and Lee University.