The report warns power stations are among the facilities at risk
Large parts of the UK's infrastructure including energy and transport networks are vulnerable to terrorism or bad weather, a report has warned.
The Institution of Civil Engineers (ICE) said not enough was being done to ensure such systems could keep going in adverse circumstances.
The ICE said efforts had been made to counter terrorism threats.
But the potential effects of climate change and system failures were not being taken seriously enough, it added.
The ICE took evidence from more than 70 sources, including regulators, agencies and service providers, and concluded that work to improve utility networks was "piecemeal".
It said when the Atomic Weapons Establishment site at Burghfield, Berkshire, flooded in 2007 all its radiation detection alarms were disabled.
Chief author of the report, Alan Stilwell, said: "It was only down to luck that the flood waters didn't lead to the spread of radioactive material that could have affected thousands of people and left the area near the factory uninhabitable for centuries."
The ICE said in the same year 350,000 people were left without water for 17 days when Mythe water treatment works in Gloucestershire flooded, while the Ulley reservoir near Rotherham came close to breaching and threatened the M1 motorway.
And last year hundreds of thousands of people were hit by electricity blackouts when Sizewell B nuclear reactor in Suffolk and Longannet coal-fired power station in Fife unexpectedly broke down within minutes of each other.
Mr Stilwell said: "We should be under no illusions, there are dangerous weaknesses in our critical infrastructure and utilities networks that need to be addressed.
"Well-defended critical infrastructure is central to the security and stability of the nation. We must work now to fortify our networks, or pay the economic, social and environmental price in the future."
The various agencies which deal with individual sectors and threats should be co-ordinated together - perhaps by a "Resilience Tsar" - to avoid future problems, he said.
He also argued that electricity and gas regulator Ofgem and water and sewerage regulator Ofwat should have more powers to help prepare for emergencies.