An investigation has pinpointed the cause of birds dying in a north London canal after widespread reports of sick swans gasping for breath.

Environmental groups raised concerns over unusual deaths of swans, geese and other wild birds reported in the north west London and Uxbridge stretch of the Grand Union Canal.

Swans were described as lying with outstretched wings, unable to lift their heads and, in some instances, gasping.

Following reports, the government’s Animal and Plant Health Agency opened an investigation to look into the matter.

It was initially suspected that it could be avian influenza, also known as bird flu, however this has been ruled out in post-mortems.

Further tests were conducted after there were theories of a toxin infecting the water, and results began to show botulism in the canal.

The toxin causes avian botulism, a disease which can cause difficulty in birds walking or moving their wings, and being unable to lift their head to eat and drink.

A generic picture of a swanA stock picture of a swan (Image: Pixabay)

According to the report, outbreaks of avian botulism can result in “hundreds of deaths”.

It is understood that dead birds have been appearing in Alperton, Paddington, and Uxbridge areas.

Swan Support, a rescue organisation, said on Facebook: “The birds affected on this stretch are not showing all the typical signs of botulism - they are going very flat before becoming lethargic and looking unwell…but they are not gasping!

“In our experience this isn’t normal - outbreaks of botulism usually follow periods of heat and extreme rainfall…

“There is no treatment as such for botulism; we give them fluids and vitamins to help flush out the toxin but it’s not always successful. 

“Unfortunately in these cases they are usually too far gone when we get called and are dying before we arrive, or in some cases, shortly after.

“We are doing all we can by removing the bodies as soon as we are aware….but beyond that we feel helpless.”

A Canal and River Trust spokesperson said: “Members of the public should not touch or pick up any dead or visibly sick birds they find, but instead immediately call the Defra helpline on 03459 33 55 77, giving as accurate a location as possible.”