The river line confirmed it had apologised to customers whose holidays "fell short of the standards we aim to deliver", after passengers on Tui Skyla reported persistent problems with air conditioning while the vessel was in Budapest during a heatwave.
Tui River Cruises said the problems affected a number of sailings on both Tui Skyla and Tui Isla, with "various unexpected technical issues" affecting air-conditioning systems.
According to the BBC, dozens of customers have accused the company of "operating vessels with faulty facilities including persistently broken air conditioning".
Tui River Cruises blamed exceptionally low water levels on the Danube and prolonged periods of extreme heat across Europe as factors that put pressure on river operations and onboard cooling systems.
Additional measures include additional cooling units and cabin fans, as well as alternative hotel accommodation and itinerary or flight adjustments where these could reduce transfer times.
"Throughout this period, our engineering teams and specialist contractors carried out extensive investigations, repairs and maintenance work," a Tui River Cruises spokesperson said.
The operator has now committed to an investment programme this winter, including "significant upgrades and replacements" to air-conditioning systems.
'Supporting affected customers is our priority'
TTG understands that following a review of its operations, Tui River Cruises has decided to appoint a new nautical and technical supplier across its river cruise operations.
The new nautical and technical supplier is also visiting ships across the fleet to support operations and ensure future technical issues can be addressed swiftly. Tui River Cruises, however, maintained its air-conditioning systems on both Tui Skyla and Tui Isla are currently stable.
Tui said its priority had been to support affected customers, keep them informed and provide compensation "in line with the level of disruption" experienced.
"We understand the disappointment and frustration caused to affected customers," the Tui River Cruises spokesperson added. "Tui River Cruises is investing in its onboard air conditioning systems across the European river cruise fleet this winter."