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.
"For years, the Congress government has used the CBI with a political motive. The BJP is following that sickening path.
"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.
According to CBI spokesperson Kanchan Prasad, the MPs allgedly claimed full payment on companion air tickets from Rajya Sabha Secretariat using fake e-tickets.
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.
They said the difference of fare on companion ticket allegedly was a benefit to the concerned MP.
The sources said the case triggered from the interrogation of JD(U) MP Anil Kumar Sahani against whom a case was registered by CBI in November last for allegedly presenting fake bills to claim the reimbursement.
They said Sahani had claimed that the practice was purportedly rampant among parliamentarians then why was he being singled out for the alleged offences.
This testimony triggered the agency to collect documents from the Rajya Sabha Secretary, the scrutiny of which showed instances where such fake inflated bills were submitted.
"In these six cases, there were repeated instances of submitting fake bills hence we registered cases against them. We have, so far, given benefit of doubt to instances where MPs have submitted just one such bill," an official said.
The sources said FIR has alleged Madani presented nine such tickets for reimbursement worth Rs 5.80 lakh whereas actual payment to Air India was only Rs 6,000. Similarly, Lalhming Liana presented seven bills worth Rs 3.14 lakh for which only Rs 26,000 were actually paid to Air India.
They said it is alleged Singh presented six tickets worth Rs 1.97 lakh whereas actual payment was Rs 6,500, Bandopadhyay presented four tickets worth Rs 1.28 lakh with actual payment of Rs 2,000, Pradhan presented four bills worth Rs 1.62 lakh with actual payment of Rs 5,600 and Pathak presented eight bills worth Rs 2.20 lakh while Rs 10,000 were actually paid to Air India.
The sources said CBI will soon ask the MPs and former MPs to appear for questioning.
They said searches were carried out at 10 places in Delhi and Odisha at the premises of politicians and some travel agents who have been named in the case.