Councils have been targeted in nearly 100 million cyber attacks in five years, an investigation has found.
Hackers and other criminals are bombarding local authorities' IT networks at a rate of 37 times every minute, figures suggest.
The findings lay bare the vast scale of the cyber threat faced by town halls, which store data relating to millions of residents.
Research by privacy campaign organisation Big Brother Watch found that councils in the UK were subjected to at least 98 million cyber attacks between 2013 and 2017.
This figure covers malicious attempts to damage, disrupt, or gain unauthorised access to computer systems, networks or devices.
The analysis found cyber attacks on local authorities most commonly involve viruses and other malicious software, or phishing, where the perpetrator attempts to obtain sensitive information such as passwords or credit card details.
Over the five years there were 376 cyber security "incidents" - cases where there is an actual breach - affecting more than a quarter of councils, according to the study.
The investigation found 25 councils experienced one or more cyber security incidents resulting in the loss or breach of data.
But the report claimed more than half of the authorities affected in those cases did not report them to police or other agencies.
Details obtained through Freedom of Information requests also suggest scores of councils do not provide mandatory cyber security training for staff.
Jennifer Krueckeberg, lead researcher at Big Brother Watch, said: "With councils hit by over 19 million cyber attacks every year, one would assume that they would be doing their utmost to protect citizens' sensitive information.
"We are shocked to discover that the majority of councils' data breaches go unreported and that staff often lack basic training in cyber security.
"Local authorities need to take urgent action and make sure they fulfil their responsibilities to protect citizens."
The scale of the cyber threat was underlined earlier this month when figures revealed Britain's defences were repelling millions of attacks every month.
A recent report from the National Cyber Security Centre detailed how local councils are among the bodies most commonly featuring in fake emails designed to trick citizens into believing they come from a trusted source so they hand over passwords and personal data.
A Local Government Association spokesman said: "Whether they are council, government or business owned, websites are being constantly bombarded by cyber attacks every single day, and protecting against and responding to attacks is a part of everyday digital life.
"Very few of these attacks actually manage to breach the firewalls or scanning systems in place, and councils are working closely with the National Cyber Security Centre to make sure that their systems and processes are as robust and resilient as possible.
"When it comes to data protection, local authorities take their responsibility to keep their residents' data safe extremely seriously.
"Councils will undertake whatever measures are necessary to keep residents' data safe and the LGA has been helping councils to adopt 'secure by design' approaches to new systems and services."