CBI sources said cases have been registered against three sitting MPs -- D Bandopadhyay of Trinamool Congress (TMC), Brajesh Pathak of BSP and Lalhming Liana of Mizo National Front--for allegedly submitting fake bills.
Former Rajya Sabha members JPN Singh (BJP), Mahmood A Madani (Rashtriya Lok Dal) and Renu Pradhan (Biju Janata Dal) too have been named as accused.
Defending Bandopadhyay as an MP with "impeccable record", TMC spokesperson Derek O'Brian condemned the CBI action saying the agency was used with the motive of political vendetta against the main Opposition parties.
"We, the All India Trinamool Congress, will fight this in the way we have fought our battles for the last 35 years - politically and democratically," he said.
Liana also refuted allegations against him saying they must be due to some "misunderstanding".
Pradhan told reporters in Bhubaneshwar that she would meet CBI authorities on Monday.
"I don't know anything about the LTC scam. I am going to Delhi on Sunday and will meet the CBI authorities on Monday," she said.
The sources said MPs booked their tickets under an Air India promotional scheme where full fare economy class ticket passengers were entitled to have one ticket for their companion on nominal rates but submitted e-tickets of full fare for claiming reimbursement.
