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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Members of the RMT union will walk out for 24 hours from 12 noon on November 7 following industrial action last month.
A Grenfell survivor told an inquiry it is a "slap in the face" for firefighters to suggest they would change nothing about their actions.
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