Visitors have stayed away from major tourist attractions in the wake of recent terror attacks, according to the London Eye and Madame Tussauds owner.
Merlin Entertainment, which also runs the Legoland theme park, also warned that numbers could continue to fall.
Bosses said before the attacks foreign tourists had been flocking to the UK to take advantage of the weak pound.
The company's London Eye sits just next to Westminster Bridge - where the March attack took place.
Merlin said in a statement: "in the immediate aftermath of the Westminster attack on 22 March... the incident did result in a softer domestic, day-trip market.
"However, the subsequent attacks in Manchester and London over the past month have resulted in a further deterioration in domestic demand and, given the typical lag between holiday bookings and visitation, we are also cautious on trends in foreign visitation over the coming months".
In the aftermath of a terror attack, major attractions tend to not feel the impact for a few weeks or months because most foreign visitors tend to buy their tickets for Merlin sites such as Alton Towers or The London Dungeons, in advance.
Chief executive Nick Varney added: "The impact of recent terror attacks on our London attractions is unclear at this stage.
"What is clear however is that London has bounced back before, and will do again.
"London is very much open for business, welcoming visitors from the UK and from around the world to this exciting and vibrant city."
Merlin has seen visitors shun its attractions after major incidents before.
In 2015, the company saw a significant drop in visitors to its Alton Towers theme park after several thrill seekers were badly injured on one of the rides.
Bosses will face tough questions from shareholders on Tuesday at the company's annual general meeting at Legoland, where they will also be told about the opening of Legoland Japan and new sites across the US.