There will be 40,000 fewer seats on London routes to the Isle of Man this summer, according to TravelWatch.
The transport watchdog said that the fall was due to the planned reduction in Easyjet flights from Gatwick on peak summer Saturdays and some peak summer midweek days, as well as the loss of Flybe's Stansted service.
Flybe announced in late 2014 that there would be three new UK regional services from Stansted, with a choice of up to 64 flights a week during the 2015 summer season, including to the Isle of Man.
However, it dropped the route in March 2016.
Meanwhile, Easyjet cut back their service between Gatwick and the Isle of Man, with one flight a day on Tuesdays, Wednesdays and Thursdays from the end of June to September.
Mike Bathgate, TravelWatch treasurer, said: "The 40,000 reduction is seats in both directions, between April and September. And that takes into account British Airways increased services to London City.
"It is down to Flybe stopping the Stansted route and Easyjet cutting back in their Gatwick flights. The primary concern is the hospitality industry – the reductions is in the peak time during the summer."