About 24,000 commercial properties in London that could be turned into temporary housing or workspaces are lying empty, a report has found.
Research from Centre for London discovered the floor space of unused buildings amounted to about 1.8 million sq m (2,700 hectares) of land.
The think tank said temporary schemes like Boxparks and gardens had proven popular but there was space for more.
It has called for more flexibility to set up "meanwhile projects."
More than 20,000 commercial properties in the capital have been empty for at least six months, with 11,000 of those vacant for two years, according to Centre for London.
The think tank said space of that kind could be used for schemes such as temporary housing, charity works or retail units.
One such project has been set up by mental health charity Core Landscapes, which has created a garden on unused land beside the Royal London Hospital in Whitechapel.
The trees and flowers in the garden are all grown in containers, meaning the project has been able to move to a new area whenever required, having first begun on vacant land beside Canning Town station.
Manager Nemone Mercer said people were referred to the project by health and social care clinicians while the garden was also used by members of the community and hospital visitors.
"There is huge passion and enthusiasm for the space. People need open spaces that can actively engage with people," she said.
Another project is the Blue House Yard, a creative hub including retail units and a food and drink court which has a five-year lease to run on vacant land in Wood Green.
The scheme's project manager Alex Lauschke said the nature of the space meant it was very affordable for businesses to start out.
"The space helps start-ups to grow their businesses. Ideally they end up outgrowing the space and move to somewhere permanent," he said.
The report's author Nicolas Bosetti said he was "surprised" by the amount of vacant properties there are given the pressure on land in the capital.
"London is full of spaces, small and large, that could be given over to meanwhile uses, but are not," he said.
The think tank has called for more flexible planning laws to make it easier to set up temporary schemes, along with better information about what buildings are vacant in the capital.