The Jurassic cycad, Encephalartos altensteninii (i), was collected by Kew's first plant hunter Francis Masson in South Africa.
The plant, known as the Eastern Cape giant cycad, arrived at Kew in 1775 and has thrived in the Palm House since it was built in 1848.
Growing at an average rate only 2.5cm a year, the knobbly trunk now reaches nearly 4.5m in length and is supported by metal stilts.
A Kew representative said: "This is the first time our gardeners have repotted this Kew icon, rehousing it in a bespoke mahogany hardwood box hand-crafted on site."
The job took three months to plan and involved a 4m-high gantry to lift the one-tonne specimen. Five gardeners had to support its trunk with stilts.
Palm House keeper Wes Shaw said: "Cycads are fascinating prehistoric plants, and this one is one of the most unique plants at Kew gardens.
"This year is Kew's 250th anniversary and it is quite fitting that we re-housed one of its oldest residents in our anniversary year. The plant is the don of the Palm House."
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