Flaws still remain in the flood defences of New Orleans leaving the city vulnerable to severe storms a month before the hurricane season starts, an independent study will announce today.

The Bush Administration has spent hundreds of millions of dollars fixing 169 miles (272 kilometres) of levees damaged during Hurricane Katrina.

But a 500-page report, compiled by engineers and disaster experts at the University of California, says that fundamental design problems and the technical weaknesses of the US Army Corps of Engineers means that the flood system remains "dangerous".

At a briefing for American journalists yesterday, Raymond Seed, a civil engineering professor at UC Berkeley who led the study, said that army engineers had made mistakes analysing seven out of eight places where the levees had failed and missed a catalogue of design errors in the flood walls.

Just a month before the start of this year's hurricane season, Mr Seed said that despite promising improvements, such as the construction of two enormous gates that can block New Orleans's canals in case of flooding, another huge storm could flood the city. "It may still be a very dangerous system," he said.

Today's study comes a week before the corps presents its own final report on the flooding of New Orleans, which caused more than 1,000 deaths and nearly $100 billion in property damage. According to interim reports, the study is expected to conclude that the scale of Hurricane Katrina exposed unanticipated flaws in the levees.

But Mr Seed said the conclusion proved the inadequacy of the corps, which maintains flood defences across America. The corps "is conducting the most important engineering analysis in its history" he said, "and they got it wrong... When the entire world is watching and a city has been destroyed, you want to get it right."

One of the 11 recommendations made by the report is that the corps lose its authority over flood control in the US and be replaced by a national flood defence authority.

Mr Seed said his team found that New Orleans's levees were fatally weakened by a thin layer of clay, the consistency of jelly, in the middle of the flood walls, and the use of seashells and other weak materials, in contrast with the corps conclusion, that the walls were simply "overtopped" by rising waters.

The Berkeley team, funded by America's National Science Foundation, also blamed local political wrangles for forcing levees to be built on narrow, delicate ridges in New Orleans's canal system, meaning that the flood defences were undermined before they were even built.

The corps refused to comment on the report before it was officially released today, but a spokesman told The Los Angeles Times: "I don't think there is any question that it will be a better levee system than before Katrina hit. We have had folks walk every inch of those levees."

According to the last official update, the corps' rebuilding of the New Orleans levees is 70 per cent complete and "progressing rapidly". The US National Hurricane Centre announces its predictions for the 2006 hurricane season today.

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