According to Imagination, the purpose-built venue – which has been praised by architect Sir Norman Foster – will be "re-purposed as an exclusive innovation space for our clients and internal teams".
The space in Store Street, Bloomsbury, will close to external client bookings from September due to "increasing internal demands", said the agency.
Hewlett Packard, Samsung and Elizabeth Arden are among companies to have used the space, which includes a six-storey atrium for up to 350, and an adjacent meeting room for 30.
The fifth floor gallery is used for up to 200 guests and the space also includes a terrace for up to 50, on the sixth floor.
Around 60 external events were previously held in the gallery each year and Imagination account services director Guy Stevenson said: "We have had the pleasure of hosting numerous prestigious events since the inception of The Imagination Gallery in 1996."
He added: "The market place today is, however, very different and, as we look to the future, we need to continue to meet the needs of our clients. As of September 2012 therefore the Imagination Building will only be available for the exclusive use of Imagination’s clients and will act as an innovation showcase where they will be able to meet in a private environment. We are excited by this plan, but it will sadly mean that we are no longer available as a venue to host events."