The agreement, which could be worth around £3m, follows the end of paving company Marshalls' three-year deal.
RHS president Giles Coode-Adams said: "This partnership with M&G will support our work to help millions of people to garden, fund vital horticultural research, brighten up communities and develop the next generation of gardeners.
"RHS shows provide inspiration for people around the world. With a phenomenal response from specialist growers, top designers and leading lights*, including a show garden by M&G, 2010 RHS Chelsea will be bursting with horticultural delights and world-class displays."
Michael McLintock, chief executive of M&G, said: "We see our sponsorship of RHS Chelsea as a natural partnership. Chelsea is about achieving the highest standards and it is that approach that has driven M&G to become one of Europe's leading active asset managers."
Show garden highlights to date include the biggest ever garden by the Eden Project, the return of regular RHS best-in-show winner, Tom Stuart-Smith with a garden for Laurent-Perrier and a garden for Tourism Malaysia designed by celebrity gardener James Wong. M&G will also bring a show garden to Chelsea to celebrate its sponsorship. The final garden selection will be made on 7 December 2009.
Other show garden highlights, confirmed to date, include: Robert Myers with a garden for Cancer Research. After being unable to exhibit at the 2009 show due to the tragic bushfires in Australia, Australian nursery Flemings will be returning to the 2010 show. Thomas Hoblyn is designing another garden for Foreign & Colonial Investment Trusts. For the first time L'Occitane will also have a show garden at the RHS Chelsea Flower Show.
There will be over 100 displays in the Great Pavilion, including: Hillier Nurseries & Garden Centres with a major feature, sponsored by Scotts Miracle-Gro. Medwyn of Anglesey with a display of over 45 types of vegetables. Raymond J Evison with an exhibit of clematis cultivars. First time Chelsea exhibitor Waterside Nursery will be creating a planted pond with design features.
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