Westminster council has dropped the £4.50 entrance fee after a flurry of complaints from the first visitors to man-made hill, which temporarily shut a week after it opened on July 26.
The town hall issued refunds to people who have already climbed to the 25-metre viewing platform and has now waived the ticket price for all of August.
Stuart Love, Westminster’s chief executive, said: “We wanted to open the mound in time for the summer holidays and we did not want to disappoint people who had already booked tickets.
“We made a mistake and we apologise to everyone who hasn’t had a great experience on their visit. With that in mind we’re going to make the mound free for everyone to climb throughout August.”
Labour councillors have demanded an inquiry into how the project “so spectacularly flopped” and how the Conservative-run local authority hopes to recoup the money it spent building the artificial hill.
Labour group leader Adam Hug said: “There are some basic questions that the council needs to answer about what went wrong with its decision making and project management.
“These are questions must be answered now but Labour believes there needs to be an independent investigation of what happened to ensure this fiasco isn’t repeated.
“We would also like to know why the council leader and its deputy leader who led the project haven’t given any public explanation about what has gone wrong?”
Westminster council hoped the attraction would draw people back into the West End and Oxford Street after pandemic restriction were dropped. The £2m building cost was to be recouped from ticket sales and sponsorship deals.
But the first visitors complained they did not see the "soaring views across central London and Hyde Park" from the lush landscape they were promised.
Instead they were treated to sights of rubble, building works and scaffolding from the 25m-high viewing platform covered in brown turf and dying trees.
Mr Love added: “London’s businesses and residents have suffered through the pandemic and we built the mound as part of our bigger plan to get people back into the city and into the shops, restaurants, theatres and to see the amazing sights the West End has to offer.
“We are working hard to resolve the outstanding issues and create an attraction worthy of our fantastic city.”