A tanker carrying 29,000 litres of milk has overturned on the A37 in Somerset spilling its contents all over the road and into a nearby stream.
The accident happened at about 0725 BST on the A37 at Ston Easton between the A39 and the B3139.
Fire crews say they have pumped about 1,500 litres from Wellow Brook which flows into the River Somer.
The Environment Agency said milk could pollute the water because it strips out the oxygen.
Devon and Somerset Fire and Rescue Service said it had stopped the leak and was using two haulage tankers to decant the milk.
Police said the A37 was closed in both directions for the foreseeable future and advised drivers to avoid the area.
No-one was hurt in the crash.