A multi-billion pound plan to build a tunnel under the River Thames that will cut through mudflats and marshes has been criticised by the RSPB.
Land would be reclaimed off Canvey Island and a lagoon created
Metrotidal Ltd, which is heading an engineering consortium, announced plans on Tuesday for a £2bn to £4bn crossing between Medway and Canvey Island.
The RSPB said the construction of the tunnel could cause irreparable damage to North Kent Marshes.
Metrotidal said their crossing would have a lower impact than other schemes.
The consortium has submitted plans for a tunnel, surge-tide barrier, tidal power plant and rail and road routes to the Department for Transport and the Environment Agency.
RSPB Director for South East England, Chris Corrigan, said: "If such a crossing is needed, all the options must be considered and the environmental damage assessed.
"This scheme appears to ride roughshod over one of the most charismatic landscapes and important wildlife sites in the Gateway.
"In addition to the direct damage the plans would cause, serious questions must be asked about how a buried tube and the huge tidal lagoon would affect tidal flow and inter-tidal habitats along the length of the estuary."
Metrotidal Ltd director, Mark Willingale, said their scheme would affect wetland but their crossing, which included a flood barrier, would have wider environmental benefits.
He added: "There is not much difference between a pure throttle [tidal] barrier and a throttle barrier with a lagoon.
"We are confident that it's the best environmental solution. This is a rail led scheme. The more people that use the train and less people that use the car, the better."
He said carbon free energy from the tidal power plant could be used to power the dredgers needed to dig the tunnel and the crossing would take noise, light and fumes underground.
The tunnel would be formed by immersing concrete tubes within a trench dredged across the Thames.
Land would be reclaimed off Canvey Island, a lagoon created on the Kent side of the river and a flood barrier installed between the two.
The plans also include a new rail route for the Thames Gateway that would link into London's Cross Rail network.