Dongria Kondh, co-ordinator of the Calder Valley tree-planting group Treesponsibility, has been camped outside the City Walk base of the government agency since 8am.
The group say they are protesting over the government's "failure to investigate seemingly unconsented works" and "moorland burning" on a big grouse shooting estate above Hebden Bridge.
Hebden Bridge and Mytholmroyd were devastated by flooding during Storm Ciara and the group claim that upland mismanagement in the area is contributing to the severity of flooding in the valley during heavy rainfall.
Dongria is stationed outside the Natural England officers holding a sign which reads: 'How long must we wait before action is taken?'
She said: “Local community groups like Treesponsibility and Slow the Flow Calderdale are doing their best to reduce flood risk, but unless the large landowners are held to account our efforts will be in vain.
"Natural England either can't or won't act as a regulator for landscape management in the uplands.
"That's an important matter for communities who depend on well-managed catchments to reduce their vulnerability to flooding, and it is also a matter of public accountability - rich landowners are receiving large amounts of taxpayer funding with little or no monitoring.
"After campaigning for better upland managment in the Hebden Water catchment for nearly eight years, my patience has run out - which is why I decided that a hunger strike was the only remaining course of action."