The Environment Agency has been dealing with oil leak on the River Kennet in Reading.
It received numerous calls earlier regarding oil in the water and confirmed that around 5.5km of the river has been affected.
Swan rescue teams have been taking birds from the water into cleaner areas.
The Environment Agency says every effort is being made to find the source of this pollution which is believed to have originated in the industrial areas in south Reading.
It says it's working with the Police, Thames Water and Swan Lifeline to minimise the impact of this pollution incident.
Oil absorbent booms have been put into the Foundry Brook to try to contain the spill.
Anyone who sees a swan in distress is asked to call Swan Lifeline on 01753 859 397.