Colin Barber said water was "streaming" into Building 45
A history society claims a former top secret munitions factory could fall to ruin without vital maintenance work.
Rhydymwyn Valley Works, near Mold, is now a nature reserve, but is said to have made 40,000 mustard gas shells weekly during World War II.
The Rhydymwyn Valley History Society says blocked drainpipes have sent water "streaming" down the inside of key listed buildings.
Flintshire County Council said a site investigation would take place.
Colin Barber, the society's chairman, said four of the buildings on the site are listed.
They include Building 45, listed in 2008, where testing was carried out as part of research into building the first atomic bomb.
Hundreds of people spent WWII making chemical shells at the site
Mr Barber said aerial photographs taken four years ago showed drainpipes on Building 45 to be clear.
However, he said: "This year it's absolutely shocking.
"There is so much growth in the drainpipes the water is streaming down the inside of the walls.
"Nothing has been done on these buildings for four years.
"We're calling for maintenance to be done. No attention is being given to this."
The society is also concerned that plant growth around the base of the buildings could undermine foundations.
Although the site is owned by the Department for Environment, Food and Rural Affairs (Defra), Mr Barber said Flintshire County Council was responsible for maintaining the listed buildings.
He said he had spent several months requesting maintenance work - which he estimates would cost "a maximum of £2,500" - but had received no response from the council.
A spokesman for Flintshire council said: "The council is grateful to the Rhydymwyn Valley History Society for its diligent monitoring of this important site.
"A site investigation will be carried out by the council to assess the conservation and ecological status of the site including key listed buildings such as Building 45.
"This will also involve Defra, the body responsible for the maintenance of the site and its structures."