WASHINGTON — Racing to meet an October deadline, Obama administration officials said Thursday that they had awarded a contract worth as much as $1.2 billion to a British company to help them sift applications for health insurance and tax credits under the new health care law.

The company, Serco, has extensive experience as a government contractor with the Defense Department and intelligence agencies, and it also manages air traffic control towers in 11 states and reviews visa applications for the State Department. But it has little experience with the Department of Health and Human Services or the insurance marketplaces, known as exchanges, where individuals and small businesses are supposed to be able to shop for insurance.

Serco will help the Obama administration and states determine who is eligible for insurance subsidies, in the form of tax credits, and who might qualify for Medicaid. Tasks include “intake, routing, review and troubleshooting of applications,” according to the contract.

“This is a huge undertaking,” said Alan Hill, a spokesman for Serco’s American unit, in Reston, Va. “We have some tight deadlines to meet.”