Analysis of Civil Aviation Authority (CAA) data found the leisure carrier ranked fifth among the airlines with the longest delays from UK airports in 2025.
TUI flights left an average of 20 minutes and 24 seconds later than scheduled across all charter and scheduled services operating from the UK.
The findings come as millions of Britons prepare for their summer getaways, with punctuality remaining one of the biggest concerns for holidaymakers.
TUI's worst-performing route was Birmingham to the Greek island of Zante, which recorded an average delay of 46 minutes among services operating at least 100 flights during the year.
The airline forms part of the wider TUI Group, which also operates the UK's second-largest package holiday business. Many of its customers book flights and holidays together through the company.
However, TUI defended its approach, insisting that its focus remains on getting passengers to their destinations rather than cancelling services.
Aage Dünhaupt, Director of Communications at TUI Group, said: "We fully understand that for holidaymakers, every minute counts. Minimising delays remains a top priority, and our teams work continuously to improve operational performance across our network.
"We make a conscious choice to operate flights wherever possible, even if they are delayed, rather than cancelling them, as this ensures passengers reach their destination rather than facing the risk of extended disruption.
"If we were to follow broader industry practices of cancelling flights more readily, our punctuality ranking would improve significantly. However, this would come at the expense of our customers."
Air India recorded the poorest punctuality performance for the second consecutive year, with departures averaging 36 minutes and 36 seconds behind schedule.
Blue Islands ranked second, with delays averaging 25 minutes and 12 seconds, followed by TAP Air Portugal at 23 minutes.
Saudia took fourth place with average delays of 21 minutes and 18 seconds.
Elsewhere, several of Britain's biggest airlines performed considerably better.
Jet2, Wizz Air and Ryanair all sat around the middle of the table, recording average delays of between 15 and 16 minutes.
EasyJet emerged as the strongest performer among the major low-cost carriers, with flights running an average of 13 minutes and 48 seconds behind schedule.
British Airways also fared relatively well, averaging delays of 12 minutes and 36 seconds, while Emirates recorded 13 minutes and six seconds.
Across all 34 airlines included in the analysis, the average delay stood at 14 minutes and 48 seconds, an improvement on 2024, when average delays reached 18 minutes and 18 seconds.
CAA Director Tim Johnson acknowledged the impact delays can have on travellers.
He said: "Long hold-ups can cause significant disruption and inconvenience for passengers.
"We recognise the impact this can have, which is why there are rules in place to protect passengers. Airlines are expected to minimise the impact of delays where possible, by providing timely information and upholding passengers' rights during disruption."
Under UK passenger rights rules, travellers delayed on departing flights may be entitled to assistance including food and drink, access to communications and overnight accommodation where necessary.
Depending on the cause of the disruption and the length of the delay, passengers could also claim compensation of up to £520 if the airline is deemed responsible, such as in cases involving technical faults or crew shortages.
For holidaymakers hoping for a smooth summer getaway, the latest rankings offer a reminder that while delays remain an unavoidable part of modern travel, knowing your rights can help ease some of the frustration when plans do not quite take off on time.
