All Creatures Great and Small actor Samuel West joined the RSPB in a "once in a generation" protest against Sizewell C.
West, who played vet Siegfried Farnon in the James Herriot drama, joined protestors outside the Department for Business in Westminster.
They lobbied Business Secretary Kwasi Kwarteng to drop proposals for the EDF's Sizewell C nuclear power plant on the Minsmere reserve.
RSPB chiefs say the site will endanger more than 6,000 species, with a breeding area the size of eight football pitches lost.
The charity also estimates the plant would affect some three million fish.
At the protest, West and other volunteers read out the names of hundreds of species which they claim will be put at risk by the power plant.
The actor said: "I don't think I know anywhere lovelier. It has an extraordinary collection of habitats, internationally-important numbers of threatened species, and we're very lucky to have it.
"So if we can't protect this, then we can't protect anything."
Jeff Knott, operations director for Central & Eastern England at RSPB, said: "We haven't done this for a generation.
"This is quite unusual for RSPB to do something like this. But Minsmere has every protection under the sun, and if Minsmere can be at risk, nowhere is safe. This is our line in the sand."
The charity has insisted it remains neutral on the question of nuclear power, but on this EDF plant, Mr Knott said: "We're facing a nature and climate emergency. We need to find solutions that work for both bits of that emergency."
A spokesman for Sizewell C claimed nearly 60% of people in East Suffolk are in favour of the plans to build the plant, according to an ICM Unlimited poll.
"Our plans will lead to a 19% increase in local biodiversity," he said. "We are not building on any land owned by RSPB Minsmere. Where a small part of our boundary meets RSPB land, we are creating a new area of wetland to allow wildlife to thrive.
"We are confident our plans will not have an impact on this important nature reserve."
Mr Kwarteng will wait until July to decide whether to grant consent for EDF to build the Sizewell C plant.
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