WASHINGTON — An influential government health panel recommended on Monday that heavy smokers get an annual CT scan to check for lung cancer, a major change in policy that experts said had the potential to save 20,000 lives a year.

Until recently, the medical consensus has been that there is too little evidence to justify lung cancer screening, largely because a chest X-ray — the usual screening technique — seldom catches the cancer early enough for lifesaving surgery.

But that changed in 2010, when a large-scale clinical trial involving 53,000 patients that was conducted by the National Cancer Institute found that a CT scan, which detects much smaller tumors, could reduce mortality by 16 percent among patients at the highest risk of lung cancer. The findings provide the principal basis for the federal panel’s recommendation on Monday.

Lung cancer claims about 160,000 lives a year — more than a quarter of all cancer deaths and greater than the toll from colorectal, breast, pancreatic and prostate cancers combined. Nearly 90 percent of patients with lung cancer die from it, in part because it is discovered too late.