The company behind a £1bn power plant has been told it is taking a risk going ahead with no environment permit.
RWE npower has planning permission for the half-built 2000 MW station near Milford Haven, Pembrokeshire, but does not yet have an environment permit.
The Environment Agency has suggested it is possible it may not be granted.
RWE npower said it was granted planning permission after a "thorough" process and it was common to pursue a permit at the same time as construction.
There have been protests from environmental campaigners that the power station was granted planning permission before any environmental assessments were carried out.
The permit is the third and final piece of permission needed for the scheme and is contingent on whether the development complies with strict European environmental law.
As well as the necessary planning permission the power station already has the Section 36 agreement from the Department of Energy and Climate Change (DECC).
In December 2008 RWE also secured an abstraction licence from the Environment Agency, which allows it to draw water from the estuary.
The Environment Agency's director in Wales, Chris Mills, admitted it was "unusual" for such a huge development to go ahead without a permit.
He said: "It is unusual for the amount of development that's taking place... before there's a certainty that an environmental permit will be issued.
He would not rule out the possibility that RWE npower could complete its construction on schedule, only to be refused a permit to operate.
He said: "This is a risk RWE npower have decided to take - we have pointed this out to them."
The developer, meanwhile, played down the possibility the scheme could be derailed by red tape at the final hurdle or that it may have to make modifications after construction has completed.
RWE npower's chief technical officer in the UK, Kevin McCullough, said it was not necessary to have the environment permit before starting construction.
He told BBC Radio Wales: "There are no unusual risks that we see in building the power station here and the way we are doing it.
"In RWE we are very used to building large infrastructure projects like this all the way across Europe and including the UK."
He added: "The main permit we need is Section 36 to allow construction to start and only when that starts do we know the exact footprint and design of the technology we are going to use.
"Then we can work with the Environment Agency to tell them exactly what impact on the environment the plant will have. So it's very commion place to do it like this."
When built, the plant would generate enough energy for more than 3m homes and provide about 100 jobs, directed to the National Grid for use in homes across the UK.
The Environment Agency has pointed to the Prenergy Biomass plant near Port Talbot as the way forward. The company behind what may become one of the biggest energy from wood plants in Britain has secured an environmental permit before starting construction.
The agency says that it is in an industrial area away from a national park, and not affecting a Special Area of Conservation.
Friends of the Earth Cymru has voiced concerns the power station could damage marine life, complained to the European Commission, and asked Welsh Environment Minister Jane Davidson to call in the application.