The two Tata Group-owned airlines together have more than 5,000 pilots. The comprehensive screening will cover substances and medications prohibited under prevailing aviation regulations, the airlines said in an internal communication to pilots.

Tests will be conducted alongside training sessions at Air India's Gurugram academy, after flights at briefing centres and airline offices, or at locations designated by pilots' home bases, according to the memo.

Existing regulations require airlines to randomly test at least 10 per cent of their flight crew for psychoactive substances each year. Air India is now going beyond that requirement by screening its entire pilot workforce.

The move follows an August 4 incident involving Air India flight AI2379, an Airbus A320neo flying from Phuket to Delhi, which suddenly lost about 300 feet (91 metres) in altitude during cruise amid turbulence.

The aircraft was carrying 145 people, including 137 passengers and eight crew members. At least 24 people were injured in the incident.

The flight's pilot-in-command, whose name has not been released by the airline, tested positive for marijuana in a confirmatory drug test. The pilot had initially tested positive for a psychoactive substance, with the confirmatory test subsequently identifying marijuana, sources said.

The incident is being investigated as aviation authorities and airlines face heightened scrutiny over pilot fitness and compliance with rules governing psychoactive substances.

Air India's decision to expand the testing comes as it seeks to reinforce safety procedures and reassure passengers following the incident.

It has told pilots that the checks are mandatory.