WASHINGTON — One in five people who signed up for health insurance under the new health care law failed to pay their premiums on time and therefore did not receive coverage in January, insurance companies and industry experts say.

Paying the first month’s premium is the final step in completing an enrollment. Under federal rules, people must pay the initial premium to have coverage take effect. In view of the chaotic debut of the federal marketplace and many state exchanges, the White House urged insurers to give people more time, and many agreed to do so. But, insurers said, some people missed even the extended deadlines.

Lindy Wagner, a spokeswoman for Blue Shield of California, said that 80 percent of those who signed up for its plans had paid by the due date, Jan. 15. Blue Shield has about 30 percent of the exchange market in the state.

Matthew N. Wiggin, a spokesman for Aetna, said that about 70 percent of people who signed up for its health plans paid their premiums. For Aetna policies taking effect on Jan. 1, the deadline for payment was Jan. 14, and for products sold by Coventry Health Care, which is now part of Aetna, the deadline was Jan. 17.