Environmental groups have welcomed a new report which could add £2 billion to the cost of building a new airport in the Thames Estuary.
The government's Airports Commission has published a feasibility study into the cost of relocating wildlife. A new four-runway hub, nicknamed "Boris Island" is backed by London's mayor who opposes further expansion at Heathrow.
The Airports Commission says the mudflats and salt marshes in the Thames estuary are home to "internationally important bird species". New road and rail links to the airport would inflict additional damage on wildlife habitats.
The report concludes the estimated £2.04 billion of moving the wildlife is "technically possible" but unprecedented in this country.