Dead fish washed up on the Broads. Photo Dennis Willis.

The Environment Agency said it had raised its barrier at Potter Heigham to protect thousands of fish that over-winter in the boatyard area there.

However, many stayed lower down the rivers and were caught out by the surge of seawater on the incoming tide.

Strong winds prevented seawater from going back out to sea, causing salt levels to rise killing the fish.

Worst disasters

The agency says it is one of the one of the worst incidents of fish deaths from natural causes in the Norfolk Broads in the last 15 years.

Spokeswoman Alison Bramwell said: "We are still assessing the full extent of the problem and are doing everything we can to help but a huge number of fish are affected."

The flooding could also affect the number of rare birds in the Broads.

A bird site in the Strumpshaw fens is under 1m (about 3ft) of salt water.

The tidal surge sent salt water up the River Yare. It went over the banks and flooded the Royal Society for the Protection of Birds (RSPB) reserve.

Suffolk's coastal nature reserves were also hit by the high tides.