Harvard Pilgrim Health Care, expanding a seven-year-old partnership with United HealthCare Services Inc., will use the national health insurer’s network to offer Harvard Pilgrim health coverage to employers with offices outside Massachusetts, New Hampshire, and Maine.

Under the joint program, called Access America, multistate businesses based in the three New England states where Harvard Pilgrim does business will be able to offer their workers at plants, sales offices, or satellite sites across the country the same Harvard Pilgrim insurance products that employees in their home offices receive.

Employees in other states are limited to United products. In the future, however, their employers will have the option of offering United or Harvard Pilgrim products, though the out-of-state accounts will use United’s national network of doctors and hospitals.

“What we’re really going to do for the first time is to compete very aggressively with the BlueCard,” said Roberta Herman, senior vice president and chief medical officer of Wellesley-based Harvard Pilgrim. BlueCard is the program used by rival Blue Cross Blue Shield of Massachusetts to sign up large national employers by referring health insurance business at their far-flung locales to sister Blue Cross Blue Shield plans in those states.

Tufts Health Plan, a Massachusetts-based nonprofit insurer like Harvard Pilgrim and Blue Cross Blue Shield, has its own alliance with national for-profit insurer Cigna Corp. Under that arrangement, Tufts’ multistate customers in Massachusetts and Rhode Island, the two states where it does business, use Cigna’s physician network for out-of-state sites.

More than 2 million people have insurance coverage under the existing partnership between Harvard Pilgrim and United. The expanded alliance has the potential to boost those numbers, said Ben Goldstein, spokesman for United, based in Minnetonka, Minn. Under the new program, United insurance programs will be offered to New England customers with Harvard Pilgrim products.

United “is excited to build on its valuable partnership with Harvard Pilgrim Health Care, which gives us the opportunity to expand the number of Massachusetts residents and employers that we serve [and] provide the marketplace with United Healthcare’s products,” Goldstein said.

Sharon Torgerson, spokeswoman for Blue Cross Blue Shield of Massachusetts, said it wasn’t worried about Harvard Pilgrim’s expanded alliance. Blue Cross Blue Shield, capitalizing on its ties with affiliates, has the largest share of national insurance accounts in the Massachusetts market.

“They’ve been trying to compete with the BlueCard for several years,” Torgerson said of Harvard Pilgrim. “We welcome the competition.”