Launched by Hodgson Events, the Modern British Art Fair will run over the same dates as 20/21, from September 7-11, at the Royal Geographical Society in Kensington Gore, London.
The announcement was made at the recent Works on Paper Fair (February 11-14), also organised by Hodgson Events and held for the first time this year at the Royal Geographical Society, a short walk from 20/21's venue, the Royal College of Art.
Lucy Russell and Richard Hodgson, co-directors of Hodgson Events, told ATG that they decided to launch the event after being approached by several 20/21 exhibitors requesting them to launch a replacement fair.
"A lot of them had already bought stock for September," said Russell. "Many of them operate without a shop and were wondering what to do."
"The Works on Paper Fair was the first time the building had been used for an event of that scale, and it proved to be very popular with both visitors and exhibitors."
Hodgson Events have secured an expanded space for the September event.
It will use the glass pavilion on Exhibition Road, linked via a temporary structure on the terrace behind the rest of the building.
The lawn will act as a sculpture garden while the Ondaatje Theatre offers space to run a talks programme.
There will be room for around 50 stands, offering British art from the 20th and 21st centuries, including paintings, drawings, original prints, photography and sculpture.
The organisers said they "hope to fill the significant gap that has been left by the hugely respected fair" run for 28 years by Gay Hutson and Bunny Wynn, who will not be involved in the new event.