Merlin Entertainments, operator of tourist attractions such as Madame Tussauds waxworks, warned on full-year earnings on Tuesday, blaming a dip in trading in its key summer period on a series of attacks in Britain, sending its shares tumbling.
The stock fell up to 21 percent after Merlin forecast core earnings for 2017 in the range of 470 million pounds to 480 million pounds ($622-$636 million).
Analysts on average had forecast 490 million pounds, according to Reuters data. The firm made core earnings of 433 million pounds in 2016.
Merlin, which also runs the London Eye, Legoland and theme parks such as Alton Towers in Britain, said that trading in recent weeks had remained mixed and group like-for-like revenue growth for 2017 was now expected to be "approximately flat" on 2016.
"The spate of terror attacks witnessed in the UK marked an inflection point in Midway London and UK theme park trading," said Chief Executive Nick Varney.
"Poor weather in Northern Europe and extreme weather in Italy and Florida also impacted peak season trading."
Britain has seen five attacks since March, described by police as terrorism.
The current official security threat level in Britain is "severe", meaning that more attacks are considered highly likely.