BBC director-general Tony Hall said the show was a major highlight of the BBC’s spring schedule.
"I’m delighted that we will be working with the brilliant RHS team for another four years. It's a show that is loved by the public and its natural home is on the BBC. Not only that, the BBC will also expand its coverage to include other RHS events around the country," he said. "Across our channels we will be able to bring the magic of these wonderful events to audiences across the country."
The deal includes a commitment to broadcasting exclusively from the RHS Malvern Spring Festival, RHS Chatsworth Flower Show, RHS Hampton Court Palace Flower Show and the RHS Flower Show Tatton Park. Plus it will cover the restoration and creation of the new RHS Garden at Bridgewater, as well as other opportunities to bring RHS content to wider audiences through Gardener’s World. In addition, there will be a new daytime series following Britain in Bloom, broadcasting in 2018.
With access to the Royal Horticultural Society’s Britain In Bloom Awards and its judging process, Britain in Bloom for BBC Two (6.30pm) will follow this Great British competition which is now over 50 years old.
Each episode presenter Chris Bavin will visit a different community around the UK as they prepare for judging. The series will reveal how Bloom has touched many lives, brought communities together and shaped local pride. The series is being produced by Electric Ray for BBC Two.
RHS director general Sue Biggs said: "We are thrilled to have extended our partnership with the BBC across all its platforms until 2021 and are so excited about the increased opportunities presented by the BBC’s expanded coverage of our work, which includes a new TV series on our flagship Britain in Bloom campaign.
"We look forward to working with the very talented teams across the BBC for another four years.
"The continuation of this fabulous partnership will help showcase our work to an even wider audience, supporting our vision to enrich everyone’s life through plants, and make the UK a greener and more beautiful place."
Coverage of the RHS Chelsea Flower Show is produced by BBC Studios’ Unscripted Unit and will start on May 21. BBC One programming will be fronted by Sophie Raworth and Joe Swift, with Monty Don and Joe Swift presenting coverage on BBC Two.
Nikki Chapman and James Wong will broadcast on BBC One during the daytime schedule and a range of presenters including Rachel de Thame, Carol Klein, Adam Frost and some of the newest faces to join the Gardeners’ World family including Nick Bailey, Arit Anderson, Frances Tophill and Mark Lane will also be appearing on the programmes.