PORTLAND – A year-old Maine law designed to block mail-order tobacco sales to minors is under challenge by the world’s largest package delivery company.

UPS said the law requiring that people delivering tobacco products make sure that each named buyer signs for the package and can prove he or she is 18 or older is so burdensome that the company has halted even legal tobacco deliveries in Maine.

In a lawsuit filed in U.S. District Court, UPS and other members of regional motor transport associations said such regulations should be made only at the federal level so they can operate with a consistent system in each state.

In arguments Wednesday before Judge D. Brock Hornby, Assistant Attorney General Paul Stern said the court should balance what he called a minor disruption to UPS’ business against the state’s duty to keep tobacco away from children.

Stern said UPS reported $2 billion in profits last year and the new restrictions impose only small costs on the company.

Financial information on the tobacco-delivery portion of UPS’ business is in a sealed portion of court records. Part of Wednesday’s arguments were made behind closed doors.

As states have raised taxes on cigarettes, smokers have increasingly turned to the Internet for bargain smokes. The Campaign for Tobacco Free Kids says Internet vendors sold about 2 percent of all cigarettes smoked nationwide in 2002 and that number is projected to triple by next year.

Maine is one of 16 states that have passed laws restricting home-delivery tobacco sales. Stern said the state requires online sellers to meet the same standards as store clerks who sell tobacco. Before making a tobacco shipment to a Maine address, the seller must find out the age of the buyer, and indicate on the outside of the package that it contains tobacco.

On delivery, only the named buyer can sign for the package, and he or she must show proof of age with a photo identification.

A lawyer for UPS said that other states have different rules. Some states allow any adult to sign for a package instead of the named buyer. Some regulate only cigarette sales, and exempt cigars and pipe tobacco.

The different rules create “a patchwork” system of regulations that burdens interstate businesses.