Operators could challenge Sri Lanka's decision to introduce a $50 charge for all visitors to the country from January 1, 2012.
The new Electronic Travel Authorization (ETA) replaces the free visa-on-arrival system and must be purchased online.
Sri Lanka marketing manager Nabeel Shariff defended the $50 charge, which operators and agents believe could deter British travellers already hit by Air Passenger Duty.
He said: "We were concerned, but the system is for every market, and not every market is in the same situation as the UK. It is still less than you pay for India or China. It is difficult from a UK perspective but we didn't want to discriminate between countries."
The new system will help the destination monitor visitors to the country, he added. "The government wants to track incoming tourists. The current system is archaic, there was no way of tracking who's coming into the country."
Lucy Nicholl, Explore's operations manager for Asia, said: “Sri Lankan tour operators have yet to be officially informed about the change to Sri Lanka’s visa process although the new system is currently being tested.
"The Sri Lankan government has conceded to tour operator opposition to this change in the past and the move is also likely to be challenged on this occasion.”
Nick McKay, managing director of Clapham-based agency Travel Designers, said: "I think it will affect Sri Lanka's trade. Having to pay $100 for two or $200 for a family will make people think twice."
Shariff said agents would have to contact customers already booked for Sri Lanka in 2012 and advise them to pay the fee online.
The ETA does not require any passport copies or photographs, and is valid for three months from the date of issue. The charge for transit passengers is £25.