Until now, both suspects and investigators have been free to smoke during interrogations. Beginning Feb. 15, they will have to take breaks and smoke in separate smoking areas.

Some prefectural police departments, such as the Osaka one, have already banned smoking in interrogation rooms, but they are still in the minority across Japan.

Tokyo metropolitan police officials said the move is a health measure to prevent nonsmokers being exposed to secondhand smoke, but they are also likely to have taken into consideration concerns about investigators giving cigarettes to suspects during interrogations.

Under a new nationwide system of overseeing police interrogations to be introduced in April, investigators giving cigarettes to suspects will be seen as giving them favorable treatment.

Some officials have expressed worries that the ban may cause interruptions in tension during interrogations when suspects go to smoke.

Smoking is already banned in office areas of the Metropolitan Police Department, except in designated smoking areas.

After a three-month trial period, the Tokyo police are expected to review outstanding issues before formally banning smoking in the interrogation rooms, according to the officials.