Ikea has pushed back the opening of its Oxford Street store to spring 2025 – about 18 months later than initially planned – after problems with water leaking into the basement.

The site, previously occupied by Topshop, had been scheduled to open this autumn and that date had already been put back by a full year.

The company said the latest delay came because “a need for unforeseen additional work on the lower of the two basement levels was recently discovered to control water ingress, which is now under way”.

Ikea’s parent group completed the purchase of the site, once the jewel in Sir Philip Green’s retail empire, for an estimated £378m in early 2022 but renovations have been dogged by complications.

The store’s opening is seen as a key part of hopes for a revival of Oxford Street, where visitor numbers have fallen since the Covid pandemic and a number of sites are either vacant or occupied by cheap American candy shops.

Peter van der Poel, the managing director of Ingka Investments, part of Ikea’s parent group, which now owns the building, said: “Ikea Oxford Street is a one-of-a-kind project, involving the sensitive and sustainable upgrade of a Grade II-listed building that is over 100 years old.

“Following the discovery of an unforeseen water ingress issue, we’re now taking vital steps to mitigate long-term flood risk, and to protect and future-proof this historic building for many years to come.”

He said the project, which aims to develop the former department store building into a gold-standard sustainable development, had moved “closer to the finish line” in recent months with the replacement of fossil-fuel gas-fired boilers with energy-efficient air-source heat pumps, as well as the installation of secondary glazing, the complete replacement of all wiring, plumbing and fire protection, and the conversion of the four upper floors into office space.

The building’s frontage has also been cleaned, which involved the removal this month of a hoarding featuring a giant blue Ikea Frakta bag.

Peter Jelkeby, the chief executive of Ikea’s UK business, said the store would have three floors with a total area of 82,000 sq ft (7,600 sq metres) and feature 6,000 products ranging from Billy bookcases to drinking glasses.

About half of the products at the store will be able to be carried away, while larger furniture items will be delivered directly to customers’ homes. The site is also expected to include a cafe and a range of room sets on display.

Jelkeby said: “The historic nature of the building makes its careful renovation more complex, but we want to ensure Londoners that we’re just as excited to open the store as they are to visit it. We look forward to doing so in spring 2025 and contributing to a positive future for London’s most renowned shopping district.”