The Competition Commission of India (CCI) in 2015 ordered a probe into allegations of anti-competitive practices after similar fares were being offered on certain routes by IndiGo, SpiceJet, GoAir, state-run Air India and now-defunct Jet Airways.
The CCI inquiry, which included an analysis of the algorithms airlines to determine ticket fares, found that all five airlines were working independently, the three sources said.
"No direct evidence of cartelisation was found," said one of the sources, who added the investigation also did not reveal any communication amongst airline executives to fix prices.
Budget airline IndiGo, the country's biggest carrier, said in a statement "the case is without merit" and added that it has been cooperating with the investigation.
The second-biggest airline, SpiceJet, GoAir, Air India and Jet Airways did not respond to a request for comment. The CCI did not respond to questions from Reuters.
An adverse finding could have led to a fine of up to three times the profit made in each year prices were fixed, or 10% of annual revenue, whichever is higher. Indian airlines are already grappling with slow growth in air traffic.
Details of the CCI case, which was investigated in two phases, have not been made public in line with the watchdog's practice.
The agency, whose senior members are reviewing the investigation findings, has not made a final ruling and could further extend the investigation. But two of the sources said the airlines were likely to be cleared.
The CCI's investigation arm had already produced a report in 2016 saying no antitrust practices were detected. The agency directed a further probe into fare-determining software and seat-allocation patterns, a second source said.
Antitrust lawyer Gautam Shahi, who is not involved in the case, said a finding of no antitrust misconduct would be a big relief for a sector that has faced intense scrutiny.
In a separate antitrust case in 2018, the CCI found that IndiGo, Jet Airways and SpiceJet had colluded over fuel surcharge rates on cargo flights, fining them a total of 544 million rupees (£5.85 million). The airlines have appealed that judgement.
By Aditya Kalra and Aditi Shah