The marine location flare was found on Saturday evening by a member of the public on Meal Beach, south east of the village of Hamnavoe on West Burra.
Police said the unexploded flare was believed to be highly toxic, and cordoned off the beach.
Bomb disposal experts are due to arrive at the scene on Wednesday.
Members of the public have been urged to stay away from the beach until the cordon has been removed and the area cleared.
Police have cordoned off a Shetland beach after a "highly toxic" distress flare washed up on the shore in the recent bad weather.