A dramatic new chapter in one of the nation's thorniest labor disputes was written this week when the International Longshoremen's Association regained the exclusive right to pack shipping containers within 50 miles of East and Gulf coast ports.

Work rules requiring that containers packed within 50 miles of the ports be packed by ILA members, or emptied and repacked by ILA members if packed by anyone else, took effect Monday after Chief Justice Warren Burger denied a weekend request for a stay filed by the American Trucking Associations.

Reinstatement of the rules after a two-year suspension was a major victory for the ILA over independent freight "consolidators" who assemble cargo at warehouses near the ports to be packed at lower cost by members of the Teamsters Union or by nonunion workers.

The National Labor Relations Board ruled on Feb. 28 that the work rules, which are contained in the master agreement between the longshoremen and the stevedores, are legal and could go back into effect.

But the battle--which has been going on almost since the advent of containerized freight in the l950s--is far from over:

* The freight consolidators, known as Non-Vessel-Operating Common Carriers, or NVOCCs, already have appealed the NLRB ruling to the federal appeals court in Richmond.

* The Federal Maritime Commission is conducting its own inquiry to determine if the rules violate the 1916 Shipping Act by discriminating against one class of shipper.

* Congress is considering legislation to strip the FMC of its jurisdiction.

Shipping experts estimated that up to 20 percent of all the containerized freight moving through ports on the East and Gulf coasts is affected by the work rules. They said that the ILA could gain more than 1,000 jobs in the Port of New York alone because of the change.

Raymond deMember, attorney for the National Association of NVOCCs, said that the economic impact of the rules is "hard to quantify," but that it would be "devastating" to the consolidators. He said shippers are threatening to divert freight to Canadian ports "rather than let the ILA handle it. They don't want the cargo loaded on the piers because of delays, damage and pilferage."

The key to the long dispute is the corrugated steel container in which most general cargo now is shipped. The container can be packed at a factory or warehouse, shipped by truck or rail to a port and loaded intact onto a cargo vessel.

Under the ILA's contract with the stevedores, containers filled to capacity at factory or warehouse, or small cargoes assembled into container-size lots and packed by consolidators more than 50 miles from a port, may be packed by non-ILA members, but a fee of $3 a gross ton is paid to the union when these containers are loaded aboard ship.

The 50-mile rule was forced on the shipping agents by the ILA as a job-saving measure. Without it, shippers of lots that are too small to fill a container could send their cargo to consolidators, who would put together enough to fill a container and then send the container to the pier, cutting the ILA out of that work.

According to David Siolkawski, assistant director of national port affairs for the Port of Baltimore, "What an NVOCC working here would do was contact an inland shipper and say, 'I'm off the piers, in the Inner Harbor or somewhere, so I can save you money. I'll pack and stuff the container for less labor charges.' Now they can't do that."