The Lumbricus terrestris was studied at the museum before being euthanised in "the sad bit of science".
It is now in a jar and will be "around for ever more", the museum said.
The 26g (1oz) worm was found by Paul Rees and given the moniker Dave by his stepson George.
`Incredibly fertile plot`
Natural History Museum scientist Emma Sherlock said: "I was bowled over by the size of this worm when I opened the plastic box they sent it in.
"Not only was it really long, it is almost twice as heavy as any other wild earthworm ever seen, weighing the same as a small chocolate bar.
"Dave is now preserved in the museum collections, and will now be useful to science."
She added: "Although his name on the jar is Lumbricus terrestris little George did request that his real name, Dave, be preserved forever too so on the label it clearly states `known as Dave` and that will be reflected in the museum database forever too."
Ms Sherlock said Dave was bigger than the longest Scottish worm, unearthed in 2015, which grew to 39.6cm (15.6 inches) on the island of Rum.
She said: "With worms this size Paul must have an incredibly fertile and well-drained vegetable plot with decaying matter quickly recycled back into the soil."
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