The Environment Agency has been accused of failing to carry out "urgent" improvements to a town's flood defences - nine years before a tidal surge hit.
A report leaked by an ex-employee shows the agency admitted in 2004 Lowestoft's flood risk had "largely been ignored".
Stephen Day said floods in 2013 could have been prevented if the document's recommendations had been followed.
The agency said it had "some reservations about the quality of the report and needed more evidence".
The report was leaked after East Suffolk Council approved plans for almost 1km (0.6-mile) of flood walls near the town's harbour.
Waveney MP Peter Aldous is still seeking £40m for a tidal surge barrier on the outer harbour.
Mr Day, who worked as a civil engineer for the Environment Agency, said his 25-page report outlined a £7m programme of flood defence works which he concluded were "urgently needed".
He said: "If they had undertaken the feasibility study within the five years as I had recommended the defences could've been built before 2013 and people would've not been flooded."
The agency said the 2004 pre-feasibility report for Lowestoft "was not suppressed" and more evidence had been needed "to support its findings".
A spokesman said a follow up report published in 2008 showed the options proposed in the earlier study were "unlikely to be publicly acceptable or economically viable".
He said at the time of the later report, the agency "could not justify, in economic terms, constructing flood walls around the harbour".
But after the 2013 tidal surge, it worked with Waveney District Council (now East Suffolk Council) to provide a temporary barrier solution for the harbour.