When California’s air regulators approved new car-pollution rules to cut down on smog and global warming, they included a provision that critics described as a loophole that could substantially reduce the number of electric vehicles sold in the state in coming years.

Under the new Advanced Clean Cars Program rules, approved last month, automakers will be required, beginning in 2018, to sell an escalating number of automobiles and light vehicles in California that can run on electricity, fuel cells or other zero-tailpipe-emission technologies.

As a result, state officials calculated that 78,100 such vehicles would be sold in California in 2018, rising to 163,600 by 2021.

But under a deal reached by automakers, the Obama administration and two senior California Air Resources Board officials, manufacturers that exceed new federal fuel-efficiency standards, even slightly, will be allowed to reduce the number of zero-emission vehicles they sell by up to 50 percent in 2018, a reduction that will drop to 30 percent by 2021.