LONDON, Sept 19 (Reuters) - Travel group TUI on Tuesday confirmed its full-year results were on track to "increase substantially," citing a strong summer and an increase in winter bookings, easing investor concerns bout the impact of extreme weather.

The news confirmed that the post-pandemic travel boom was still going strong, sending TUI's shares up 6.5%, and on course for their biggest gain since March.

European airlines have also benefited from a bumper tourist season across the continent, but extreme weather ranging from heatwaves to flooding had dampened some of the optimism about a strong travel rebound.

Summer bookings for 2023 were at 13.7 million, up 5% over the previous summer season, the company said in a statement. TUI shares were among the top gainers on the STOXX index on Tuesday.

"Had it not been for the various events during the last few months which were outside of our control, not least the wildfires on Rhodes, we would have performed ahead of expectations," CEO Sebastian Ebel said in a statement.

Forward bookings were up 15% against winter of 2022 and 2023, the company said, while summer bookings were at 96% of pre-pandemic levels.

Even as Europe battled extreme weather that sparked unprecedented levels of disruption, the company said it was on track to reconfirm its underlying earnings before interest and tax would increase substantially for the fourth quarter and the year, with continued growth set for 2025 and 2026.

The share rise showcases investors' optimism after strikes and staff shortages weighed on the travel sector as it recovered from the pandemic, analysts said.

"I think the positive reaction here is to a clean set of results, with signs of acceleration into October and winter," said Richard Clarke, an analyst for Bernstein.

TURBULENT SUMMER

Eurocontrol said in July that weather-related air travel disruption was two and a half times higher than in 2022, with thunderstorms being the largest cause of delay.

Last week, TUI said it was monitoring adverse weather conditions in its key destinations. Earlier, it forecast losses at up to 25 million euros due to wildfires on the Greek islands.

Strikes and a technical glitch in air traffic management inputs in Britain also disrupted travel this summer, adding more concern that consumer appetite for airlines like TUI and Ryanair might wane into the winter.

(Reporting by Joanna Plucinska; editing by Louise Heavens and Jason Neely)