Scientists say a number of factors may be responsible for the decline
Eel populations in the River Thames have fallen by 98% in just five years, scientists have said.
The numbers of eels - which have been a traditional east London dish for centuries - fell from 1,500 in 2005 to just 50 last year.
Conservationists fear this could have a knock-on effect for other species in the river's ecosystem.
The Zoological Society of London records the numbers by capturing eels in traps before releasing them.
They are thought to take up to three years migrating as larvae from the Sargasso Sea to European rivers, where they spend up to 20 years before making the 4,000 mile (6,500km) return journey across the Atlantic to spawn and die.
Conservationists are concerned the species is not returning to the Thames, or is facing problems in the river and its tributaries.
European eels and flounders were the first species to recolonise the Thames Estuary after being considered "biologically dead" in the 1960s.
Dr Matthew Gollock, tidal Thames conservation project manager at ZSL, said: "Eels are mysterious creatures at the best of times but we are very concerned about the rapid disappearance in the Thames.
"It is difficult to say what is going on - it could be due to a number of potential factors including changes in oceanic currents due to climate change, man-made structures such as dams and the presence of certain diseases and parasites."
And he said there was a need to find out why the declines were happening, in order to save the eels and help other species in the estuary's food web which would be affected by their disappearance, such as birds which feed on it.
"Time appears to be running out for eels in the River Thames and this could have a domino effect on other species in the Thames," Dr Gollock added.
Jellied eels have been eaten since the 18th century, most famously in the east end of London.
The dish, served in Pie and Mash shops, is made up of chopped eels, boiled in a spiced stock that is allowed to cool and set, forming a jelly and can be eaten hot or cold.
Most eels used in Pie and Mash shops now come from Holland or Northern Ireland.