The Crawley based group said yesterday that it had incurred a cumulative cost of £20m from Thursday to Sunday last week and estimated that daily costs thereafter fell to between £5-6m a day.
TUI said that approximately 100,000 customers are stranded, while 90 per cent of customers due to travel have decided to re-schedule.
TUI said yesterday that it was supporting stranded customers by footing the bill for extra hotels and food.
The Icelandic volcano, which has disrupted travel since Thursday, has taken its toll on the wider travel industry, with airlines and airports reporting significant financial losses.
The Airports Council International (ACI) has said the impact is worse than the subsequent cancellations experienced after the 11 September attacks on New York.
The European airport body said that 313 airports are paralysed by the grounded flights and have already lost €136m (£119.6m).
ACI director general for Europe Olivier Jankovec said: “While safety remains a non-negotiable priority, it is not incompatible with our legitimate request to reconsider the present restrictions.”
ACI reported that 6.8m passengers were affected by the grounded flights.