London Zoo says it believes Melati carried the cub outside, but keepers are unclear as to how the cub got into the pool.
Keepers are now reviewing the situation because it was not forseen that the mother would take the cub outside so early.
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High level of cloud will turn this morning's sunshine increasingly hazy. Tonight will stay dry with clear spells.
The eight-week old British Bulldog puppy is so valuable because of its rare colouring.
E-cigarettes have been banned on London buses. Smokers who switch to electronic devices are already prevented from using them on the tube.