TUI owns First Choice in the UK
Europe's biggest travel operator TUI Travel has said the disruption caused by the volcanic ash has cost the group about £20m ($30m; 23m euros) so far.
TUI added that daily costs were running at about £5m to £6m.
The group said that as of Sunday, it had about 100,000 customers who had been unable to return home.
TUI said it was working with other tour operators to ensure that regulatory authorities and governments permit flights to resume as soon as possible.
TUI owns Thomson and First Choice.
Paul Bowtell, TUI's chief financial officer, was critical of the government's response to the crisis.
"I think the government has been very slow," he told the BBC. "Gatwick Airport on Thursday was still saying it hoped to have flights throughout the day.
"Now we're five days on and we still haven't got any clarity regarding what the government is doing to get airspace open."
Calls for review
The International Air Transport Association has estimated that airlines alone will lose at least $200m a day in revenues as a result of the disruption.
But the air industry has called for an urgent review of the current flight bans, questioning the need for the unprecedented curbs.
Some airlines, including British Airways, KLM, Lufthansa and Air France, have carried out test flights and say planes showed no obvious damage after flying through the ash.
EU transport ministers are to hold emergency talks by video conference later, aimed at easing the crisis.