Water minister Richard Benyon has called for a clampdown on a pesticide that "seriously" polluted the River Kennet between Wiltshire and Berkshire.
The MP said an "egg cup" of Chlorpyrifos affected a 10-mile (16km) stretch of the river, killing shrimp.
Mr Benyon said the "fragile eco system" of the river had been "devastated". He added the tiny amount of pesticide that caused the damage was "terrifying".
The Environment Agency is investigating the source of the pollution.
'Nothing to eat'
Chlorpyrifos is routinely used as an insecticide in agriculture, in fruit farming and at municipal parks and golf courses.
Freshwater shrimp, mayfly and caddis fly in the river appeared to have been wiped out by the pesticide, "leaving birds and fish with nothing to eat", an Action for the River Kennet (ARK) spokesman said.
The pesticide entered the Kennet via a sewage pumping station in Marlborough, Wiltshire and affected wildlife downstream in Hungerford, Berkshire.
Only professional people are authorised to buy and use it.
Mr Benyon, who lives by the River Kennet in Newbury, has called on Defra to look into imposing more restrictions.
"The worry is that this chemical is purchasable online," he said.
A Defra spokesperson said: "When used wrongly, pesticides can have devastating effects on wildlife, as happened here".
She added it was working with the Environment Agency to investigate the incident.
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