BRUSSELS — The European Union began a broad investigation Thursday into whether Chinese companies have exported solar power equipment for less than the cost of making it, in what amounts to history’s biggest anti-dumping investigation by value.

The case covers imports from China worth €21 billion, or $26.5 billion, last year, a hefty 6.5 percent of all E.U. imports of Chinese goods. The European case is four or five times larger by value than a similar investigation under way in the United States, because the Union is the biggest export market for Chinese solar panels.

The European Commission, which is leading the investigation announced Thursday, said the Union accounted for 80 percent of Chinese solar equipment sales worldwide.

After exerting heavy diplomatic pressure for the Union not to start the case, and after veiled threats to retaliate, the initial Chinese response Thursday was comparatively restrained.