“China is offering the financing, the building, the equipment—even the labor,” said Jörg Wuttke, president of the Beijing-based European trade group. “That is a very closed system.”

China has framed the Belt and Road initiative as a way to more closely integrate its economy with those across Asia, Africa, Europe, the Middle East and Latin America, primarily through infrastructure projects. That has earned it comparisons to the Marshall Plan, under which Washington gave billions of dollars to help rebuild swaths of Europe and which placed the U.S. at the center of the postwar economic order.

During the first 11 months of 2019, Chinese companies signed 6,055 contracts in 61 countries as part of Belt and Road with a total value of $127.67 billion, an increase of 41.2% from the same period a year earlier, according to China’s Commerce Ministry.

The Belt and Road effort, however, has attracted criticism alleging that it lacks of transparency on financing, leading to charges of corruption, waste and out-of-control debt. President Xi Jinping has responded in part by recalibrating his signature program, pledging more transparency and financial sustainability.

However, the European Chamber, in its report published Thursday, said some apparently successful Belt and Road projects relied on large Chinese subsidies to stay afloat, quoting one logistics company that detailed handouts that heavily distorted market prices for freight traffic between Western China and Europe.

The trade group counted that just 20 of its 1,700 member companies in China had participated in the initiative. Those that did so joined because of existing relationships with the Chinese government or to lend technology expertise that couldn’t be supplied by Chinese companies, the chamber said.

Most of the participating companies did so through joint ventures with Chinese state-owned companies and in almost all cases held a small minority share in the project. Others had bid on Belt and Road-related projects, though very few were awarded the project, said the report, which pointed out the lack of an office or even a phone number for the initiative.

Geng Shuang, a Foreign Ministry spokesman, dismissed the European Chamber’s report as biased, saying the initiative was launched in a spirit of openness, inclusiveness and transparency.

Under the Belt and Road initiative, “Chinese and foreign companies follow the market rules and principle of fairness,” Mr. Geng said Thursday. He listed Germany’s Siemens and Deutsche Post’s DHL business, as well as France’s Schneider Electric, as examples of participating European businesses.

Siemens, which was among the first Western companies to work with Chinese companies on Belt and Road projects, opened an office in 2018 to oversee its involvement in the plan.

While some U.S. companies have shown interest, few have gotten directly involved. Most notable among them is General Electric Co., GE -0.25% which has partnered with Chinese state-owned enterprises on power-plant and grid projects in Africa. In July, Larry Culp, GE’s chairman and chief executive, met with Hao Peng, chairman of the body that oversees China’s state-owned assets, to discuss the Belt and Road initiative.

Some of the Belt and Road’s advertised participants had been only retroactively labeled as joining the effort, the European Chamber said. When a Chinese consortium won a bid in 2018 to build a bridge in Croatia as part of an EU-funded initiative to create transport, energy and digital connections between Europe and Asia, the Chinese news media were quick to laud it as a part of the Belt and Road initiative.