The employees can book tickets by utilizing the services of authorised travel agents-- M/s Balmer Lawrie & Company, M/s Ashok Travels & Tours, and IRCTC, it said.
The directive came after it came to the notice of the government that in a number of cases these instructions were not being followed by the employees.
CBI is probing fake travel bills scam (LTC scam) allegedly involving certain government employees and present and former Members of Parliament.
"The most common reasons given by the employees are unawareness of the rules and non-availability of authorized travel agents viz M/s Ashok Travels, M/S Balmer Lawrie & Company at places where the tickets have been booked from.
All the ministries or departments of the government are advised to ensure that their employees are made aware of these guidelines to avoid breach of any of the LTC rules, it said.