The deputy leader of a London council has resigned after the total costs for a 25-metre human-made mountain in the middle of one of the capital’s busiest tourist areas nearly doubled to GBP6m.

Refunds were offered to customers of the Marble Arch Mound, created by Westminster city council, the day after it opened to members of the public on 26 July after what the authority called “teething problems” with the attraction amid complaints it was still being constructed.

The council’s leader, Rachael Robathan, said in a statement on Friday her deputy, Melvyn Caplan, had resigned with immediate effect after a “totally unacceptable” rise in costs.

“The mound opened too early, and we have apologised for that,” Robathan said.

“It has become clear that costs have risen more than anticipated and that is totally unacceptable. Our original forecast cost was GBP3.3m. Total costs are now GBP6m, covering every aspect of the project: construction, operation and eventual removal.

“With regret, I have accepted the resignation of my deputy leader, Melvyn Caplan, who led the mound project. We have also instigated a thorough internal review to understand what went wrong and ensure it never happens again.”

The 25-metre mound, planned by the Dutch architects MVRDV, was designed to give views of Oxford Street, Hyde Park, Mayfair and Marylebone. It is part of a scheme to increase footfall in the shopping district as lockdown restrictions ease.

“We are determined to continue our hard work to restore our city’s vibrancy, bring back visitors and ensure people can keep their jobs,” Robathan said on Friday.

“Doing nothing was never an option. So when the mound fully reopens in September, I hope that people will come and see it for themselves. The mound may delight or divide views and that’s OK, but we’re confident that in the end it will fulfil its original brief – to get people back into the West End and remind them of why this is a world-class city.”

The council announced on Friday that tickets to the mound, which is due to remain open until January 2022, would be free for August.

Caplan has served as a councillor for the Little Venice Ward since 1990, with the Conservative also a former leader of the council.