A new study provides what independent researchers call the best evidence yet that colonoscopy — perhaps the most unloved cancer screening test — prevents deaths. Although many people have assumed that colonoscopy must save lives because it is so often recommended, strong evidence has been lacking until now.

In patients tracked for as long as 20 years, the death rate from colorectal cancer was cut by 53 percent in those who had the test and whose doctors removed precancerous growths, known as adenomatous polyps, researchers reported on Wednesday in The New England Journal of Medicine. The test examines the inside of the intestine with a camera-tipped tube.

“For any cancer screening test, reduction of cancer-related mortality is the holy grail,” said Dr. Gina Vaccaro, a gastrointestinal oncologist at the Knight Cancer Institute at Oregon Health and Science University who was not involved in the research. “This study does show that mortality is reduced if polyps are removed, and 53 percent is a very robust reduction.”

Colorectal tumors are a major cause of cancer death in the United States and one of the few cancers that can be prevented with screening. This year, more than 143,000 new cases and 51,000 deaths are expected. Incidence and death rates have been declining for about 20 years, probably because of increased use of screening tests and better treatments. But only about 6 in 10 adults are up to date on getting screened for colorectal cancer, according to federal estimates.