The world's largest egg, belonging to an extinct Madagascan elephant bird, is up for auction in London.

At 30 centimetres high and 21 centimetres wide, the egg is about 100 times larger than a chicken egg and larger than those laid by the dinosaurs.

It is expected to fetch up to $40,000.

James Hyslop from Christies auction house says it is uncommon to find an elephant bird eggs in such good condition.

"It was laid by a giant elephant bird that lived on Madagascar and went extinct somewhere around the 13th to 17th centuries," he said.

"It's very rare to find one that's complete. To see one in such perfect condition is a real treat."