With a section of Air India unions going on strike disrupting flight schedules, private airlines had a field day today effecting an undeclared hike of 10-20 per cent, industry officials said.

The airlines, however, denied any fare hike on their part saying the fares were always "dynamic" and based on demand and supply.

"The fares today are 20 per cent up compared to yesterday's fares, without the airlines actually resorting to a hike," Travel Agents Federation of India General Secretary Ajay Prakash told PTI.

The average fares in the airlines' system were reflecting between Rs 5,700-8000 across sectors as against Rs 4,500-6000 which were earlier showing on their websites, Prakash said.

A city-based businessman, Bholenath, had to shell out as high as Rs 30,000 for two tickets for one-way travel to Varanasi for himself and his son, after Air India cancelled its Mumbai-Varanasi flight.

"I am travelling to Varanasi with my son and the total (Air India) ticket had cost me Rs 7,000. Since Air India has cancelled my flight and I have to reach there by this evening, I approached SpiceJet but found that the fares were too high -- Rs 30,000 for both of us. But there was no option," Sharma told reporters at the airport here.

Similarly, Jet Airways website reflected a fare of Rs 13,069 in the JetKonnect economy class for a Mumbai-Jaipur journey on May 28.

Echoing similar views, Travels Agents Association of India said that the airlines took the shelter under yield management system for the "undeclared" hike.

"The yield management system automatically reflects the higher-end fares, once the seats in the lower end are filled.

And the airlines are simply taking advantage of the situation and the system," TAAI President Rajji Rai said.

While SpiceJet Chief Operating Officer was not available for comments, a Jet Airways spokesperson said that the airline did not jack up fares as dynamic fares were being reflected in the system.

"We did not hike fares. We have dynamic fares. Once the low-end basket is filled up, the system automatically goes to the higher-end and reflects those fares," the Jet Airways spokesperson said.