The pat-down may include sensitive areas such as breasts, buttocks and the groin area.

Until last week, TSA screeners could employ any of five pat-down procedures to check the few passengers who chose not to go through a millimeter wave imaging booth or walk-through metal detectors.

Now all passengers will receive the same pat down, the one previously known as the “full pat down.”

The TSA, in a statement, said that a full pat-down “does not involve any different areas of the body than were screened in the previous standard pat-down procedure.”

The new guidelines posted on the TSA website are much more explicit about what passengers should expect: “A pat-down may include inspection of the head, neck, arms, torso, legs, and feet. This includes head coverings and sensitive areas such as breasts, groin, and the buttocks. You may be required to adjust clothing during the pat-down. Pat-downs require sufficient pressure to ensure detection.”

The guidelines say that a pat-down will be conducted by someone of the same gender as the passenger and will be witnessed by a second TSA screener of the same gender.

The same pat-down will be employed with those who set off alarms in the millimeter machine or a metal detector, and those singled out by bomb-sniffing dogs.

“Even passengers who normally receive expedited screening, such as TSA PreCheck passengers, may at times receive a pat-down,” the guidelines said.

There was some political pushback to reports of the change.

Ron Paul, the Republican former congressman, tweeted: “TSA Launches ‘Invasive’ Pat-Downs With ‘More Intimate Contact Than Before.’ ” He republished an article on his website that referred to the new procedure as a “universal method which would involve heavier groping.”

Airport security was tightened in 2015 after a damning inspector general’s report that said undercover operatives were able to slip through airport security with weapons and phony bombs more than 95 percent of the time. They were able to carry weapons or bomblike materials through airport security checkpoints in 67 of 70 attempts last year.

TSA responded by putting the entire 60,000-member TSA workforce through retraining, opening a training academy in Georgia for recruits and bringing in several new top administrators. It also ended a practice in which random passengers were selected and allowed through special lines that were designed for travelers who had qualified by advance background checks for expedited screening.