It has written to Environment Secretary David Miliband, urging him to insist on a full investigation into the envionmental consequences of Essex and Suffolk Water's Abberton Expansion Scheme.
ESW wants to raise levels at Abberton to see the drought-hit South-east through dry summers.
It says it will need to increase the amount of water it abstracts from the Great Ouse at Denver, near Downham Market, to make the reservoir 60 per cent larger.
But environmental assessments show if the scheme gets the go-ahead, it could damage vital habitats, by increasing saline encroachment in the tidal Ouse, which could in turn lead to parts of the Old Bedford River, Delph and Well Creek becoming saline.
"We began investigating this when members were warned that fishing at Abberton Reservoir, in Essex, would be stopped," said PAC secretary Mark Barrett.
"We have since learned that historically-important pike fisheries are also in jeopardy, along with the biodiversity of the Great Ouse catchment - not to mention its estuary, The Wash.
"A threatened fish species, the spined loach, is also under threat. We are therefore calling for a public enquiry not only to safeguard our members' fishing, but the unique and vital habitats whose future is now in doubt."
In its letter to Mr Miliband, the PAC says:
"We enclose preliminary environmental assessments, which show that the increased abstraction of water from the River Ouse, which is part and parcel of the scheme, could reduce flows in the tidal river, increase salt encroachment and threaten both historic coarse fisheries and internationally-important bird reserves.
"We fear this scheme could alter the entire ecology of parts of the Fens, heighten risks to endangered species and threaten populations of coarse fish and birds which are already at grave threat from climate change.
"Both fish and birds play a vital role in the tourism economy of the area. Along with the thousands of anglers who travel from all over the country to fish the Fens, tens of thousands of people visit the Ouse Washes and Wildfowl and Wetland Trust reserves.
"The Abberton Expansion Scheme has potential to create controversy on both a national and international scale, since the Ouse Washes, which would be placed at increased risk of becoming saline, are a Ramsar site."
The RSPB has said the ecology of the internationally-important Ouse Washes, which are a Ramsar site for over-wintering birds, could be at risk. See here
The Pike Anglers Club fears it will also heighten threats to coarse fish populations and the biodiversity of the entire Great Ouse catchment, where fisheries scientists are already on record as saying changes in river management practices have led to whole year classes of fish being lost.
The Great Ouse Boaters Association fears increased abstraction at Denver could reduce flows in the tidal Ouse.
This in turn would increase siltation around sluices, which would mean water being let out to sea via the Relief Channel, whose fish populations have been devastated by harsh winter run-offs.
"We are a conservation group which campaigns to preserve both the pike and pike fishing," Mark Barrett said.
"We are therefore calling for a public enquiry into this scheme and the opportunity to put our case that riverine environments will be at risk if it is given the go-ahead in its present form."
The PAC is writing to MPs and conservation groups such as the RSPB, Wildfowl and Wetlands Trust, along with angling clubs across the Fens to call for their support.
==========================================What you can do:
Write to David Miliband, the Environment Secretary, urging him to call a public enquiry when ESW applies to increase its abstraction licence to enlarge Abberton.
Letters should be sent to:
The Rt Hon David Miliband MP
Secretary of State for the Environment Food and Rural Affairs
17 Smith Square
London SW1P 3JR
More information on the scheme and it's possible impact on the Fens are on ESW's website - the preliminary environmental studies can be viewed and downloaded here: