United Airlines flight from San Francisco to Frankfurt is declaring an emergency and diverting to London.
The crew declared an emergency at 1:pm BST (squawk 7700).
The flight is operated by a Boeing 777-300ER (registration N2737U).
UPDATE 13:12 BST
On approach to runway 27L.
UPDATE 13:21 BST
Flight #UA926 has safely landed on runway 27L at London Heathrow.
UPDATE 13:23 BST
Going to terminal 2 gate B46.
UPDATE 14:56 BST
Flight #UA926 is resuming the flight to Frankfurt following the medical emergency.