NEW YORK (MarketWatch) -- The stalled contract negotiations between West Coast dockworkers and port owners has many stakeholders on edge, including Japanese fans of McDonald’s fries.

McDonald’s Japan in mid-December began to ration its fries to small sizes, causing a local consumer outcry, because the slowdown at the ports delayed the import of potatoes.

On Friday, 174 U.S. trade groups, representing retailers, manufacturers and farmers, sent a letter to the International Longshore and Warehouse Union and ports operator, Pacific Maritime Association.

The letter came after both parties accused the other side of causing, in PMA’s words, something “approaching complete gridlock ” at the five largest ports on the West Coast, the main processor of U.S. import and export cargos.

While both sides agreed to the involvement of a federal meditator in early January, their latest public salvo raised fears that the ports are heading toward a shutdown, which could cost the economy $2 billion daily if it lasts 20 days, according to Jonathan Gold, National Retail Federation’s vice president for supply chain. Trade through the 29 West Coast ports represents 12.5% of U.S. GDP, Gold told MarketWatch.

The impact “could be catastrophic,” he said.

The slowdown that began in the fall has already wreaked havoc. Retailers have told NRF that it now takes them anywhere from 7 to 10 days to weeks to get goods into their local warehouse once ships have been docked. Typically, it would take only two to three days, Gold said.

“Our members desperately need this negotiation to be concluded and operations returned to normal levels,” the letter representing the trade groups said. “It is a black eye for the broader economy and some jobs have and will continue to be lost as a result.”

Manufacturers in the Midwest have had to slow and even halt production due to delays in receiving components, the letter said. Retailers are experiencing delays in early spring merchandise, including products for Valentine’s Day and Easter.

The latest ISM report released this month also mentioned the port issue. Fabricated metal products makers, for instance, have flown parts in from Asia to support production while their parts sit on the dock. Textile mills are also reporting delays in receiving imported goods.

Even the Federal Reserve’s Beige Book released this week mentioned the port issue. It cited a Boston manufacturer’s export being hurt, while a toymaker said delays in getting products has hurt sales.

Retailers from New York & Co. US:NWY to Steve Madden Ltd. SHOO, -1.94% this month joined Lululemon Athletica Inc. LULU, -2.75% , Ann Taylor parent Ann Inc. US:ANN and GameStop Corp. GME, +2.93% in singling out the issue. Some have cut sales or profit forecast after having to fly products in.

A Cowen report on Thursday said that while lower gas prices could boost consumer spending, retailers face increased shipping costs due to the port slowdown.

The impact on the agricultural sector also has been dramatic, hurting producers from Washington apple growers to Idaho potato farmers.

McDonald’s Japan resumed the sale of normal portions on Jan. 5, after flying in potatoes or sending shipments from the East Coast.