The Gillespies-designed space at the top of the Walkie Talkie building - 20 Fenchurch Street - in the City of London was billed as a new public park while in the design and planning stages. CGI images showed a park-like environment with trees and open views.
But with restricted opening hours, airport-style security, a series of strict conditions on entry and a booking system that is no longer open for bookings, some have questioned its "public" label.
Metropolitan Public Gardens Association chairman William Fraser said he has called for planning authority the City of London Corporation to investigate after it emerged that the Sky Garden was so popular that visits to it have been booked up until the end of March.
"We too are disappointed in public opening of the gardens," he added. "We believe the current proposals on opening do not conform to planning consent and hope there will be a change."
The garden is open from 10am to 6pm on weekdays, with evenings reserved for patrons of the restaurants and bar, which take bookings up to 60 days in advance.
Free visits are also allowed 10am-10pm on weekends, when there are slots available. However, because of demand, currently only those who pay for a meal or a drink in the building's two restaurants and bar can visit.
A representative of the building's owners, Land Securities and Canary Wharf Group, said the restrictions are necessary for public safety but added that the owners had no comment on the planning issue.
Compliance - Planning authority responds
A City of London Corporation representative said: "The garden has fewer trees than were shown in the images. However, for the secretary of state who granted the original permission and for the city, an important feature of the building was that public access to the view and the sky garden was to be provided free of charge to the public.
"This principle is what we have insisted is maintained as set out in a planning agreement. The developers have submitted a visitor management plan to demonstrate how they intend to meet this obligation. As the responsible local planning authority, we will seek to ensure that the developers comply with this obligation."