Container-shipping lines, already concerned about demand from struggling economies in Europe and many emerging markets, can add China to their list of problems, according to a new report from Drewry Shipping Consultants Ltd.

The consulting and research firm lowered its forecast for containers moving through ports in China and Hong Kong to grow by 4.9% this year, from 5.8% in an earlier outlook. That is the equivalent of removing 1.85 million 20-foot equivalent units, or 1% of world-wide container traffic, from ocean trade lanes.

China’s economy has been gradually slowing for several years, but warning signs of a sharper downturn are piling up.Manufacturing activity in China fell to a two-year low in July according to data reported Monday in Beijing, and economists say the country’s stock-market rout could sap consumer and business confidence.

Hong Kong has seen 12 straight months of year-over-year declines in container volumes, although some of that activity has shifted to ports on the mainland.

Dry bulk shipping has been hardest hit as Chinese factories cut back on imports of iron ore and other commodities. But container shipping lines are in for some pain as well, given that China and Hong Kong represent about 30% of global container traffic, Drewry says.

One mitigating factor: China’s slowing economy will have a greater effect on incoming traffic, which is far less profitable for shipping lines than on outbound containers, said Simon Heaney, senior manager of supply chain research at Drewry.

“For shipping lines, any decrease isn't particularly great at this moment,” he said, adding that “in the grand scheme of things…it’s a risk but a small one.”

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Despite warnings that too much capacity and weak demand are putting downward pressure on rates, some of the largest shipping lines continue to report strong results. A relatively strong U.S. economy is helping offset weakness in Europe and emerging markets. Japanese shipping lines last month reported strong profits, helped in part by a weak yen. A.P. Møller-Maersk A/S, parent of Maersk Line, the largest container shipping line, reports earnings next week.

Many analysts have adopted a bleak outlook for container shipping. Giant new vessels capable of carrying around 20,000 TEUs are hitting the market even as global volumes underwhelm.

“There are far too many ships at the moment searching for too little cargo,” Mr. Heaney said. “That’s unfortunately a situation the industry will be plagued with for a number of years.”

Write to Brian Baskin at brian.baskin@wsj.com