An investigation launched in the wake of the Edinburgh outbreak of the illness has exposed reductions in the number of Legionella inspections carried out by the Health and Safety Executive.
The probe by the publication Environmental Health News found the number of pro-active HSE inspections across the UK fell from 833 in 2009 to 464 in 2011.
The number of Legionella inspections at cooling towers dropped from 237 in 2010 to 134 in 2011. Around 2900 cooling towers are subject to inspections from the HSE.
The source of the Edinburgh outbreak is yet to be determined but attention has centred on cooling towers in the city.
Professor Hugh Pennington, emminent microbiologist and chair of the public inquiry into Scotland's fatal Wishaw Ecoli outbreak, said: "It is clearly time for a rapid root and branch reform of the regulatory system for Legionella."
Lawyers representing victims of the Edinburgh outbreak, in which more than 100 people fell ill and three people died, expressed serious concerns about the findings.
Clive Garner, of Irwin Mitchell, said there were fears that towers which could pose a serious public risk will not be inspected until it is too late.