The photograph was taken at Hornchurch Country Park in east London on Monday afternoon.
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Speaking to BBC News, Mr Le-May said he had managed to capture the moment while he was out walking with his wife Ann.
He said: "I heard a distressed squawking noise and feared the worst.
"I soon realised it was a woodpecker with some kind of small mammal on its back.
"I think we may have distracted the weasel as when the woodpecker landed it managed to escape and the weasel ran into the grass."
Mr Le-May said he was astounded by the reaction to the picture on social media.
The wildlife shot has been retweeted several thousand times.
BBC