A Severn estuary barrage could increase the risk of flooding and adversely affect wildlife, the RSPB claims.
The conservation charity made comparisons to a storm surge barrier across an estuary in the Netherlands to illustrate the potential "risks".
The RSPB said the report showed that a barrage across the Severn would "devastate the estuary".
The Department of Energy and Climate Change (DECC) said it was examining the viability of a Severn tidal scheme.
The report about the barrier across the Oosterschelde showed it had led to increasing erosion and higher water levels, and could make shipping channels difficult to navigate.
RSPB conservation director Mark Avery said: "Although smaller, the Oosterschelde is very similar to the Severn Estuary in many ways and it is being damaged beyond repair."
The barrier in the Netherlands was built to prevent storm surges while the Severn barrage would use tides to generate electricity.
Mr Avery urged for other ways to generate electricity using tides.
"We have long said the government should invest in innovative schemes which offer the potential to put the UK and UK engineering at the forefront of tidal power without the risk of floods, loss of wildlife and livelihoods," he said.
A DECC spokesman said the UK government was carrying out a two-year feasibility study into whether it could support a tidal power project in the Severn estuary.
"A tidal power scheme could generate up to 5% of the UK's electricity - as much renewable electricity as we produced in 2008 across all renewable technologies," he said.
"However, we need to understand the impacts, costs and the benefits before concluding whether a scheme would be a good idea.
"In order to understand the environmental and socio-economic effects of a tidal power scheme, we are carrying out a strategic environmental assessment (SEA)."
He said the DECC was looking at what could be learned from the storm surge barrier in the Netherlands, and other estuary and coastal power generation schemes.
"In 2008, at the invitation of the Severn tidal power feasibility study team, Dutch officials gave a presentation on the construction and subsequent impacts of the storm surge barrier," he said.
"As we have previously made clear, once the evidence gathering is completed, we'll publish our findings including the detailed technical reports from the SEA as part of a public consultation on Severn tidal power."