Hundreds of personal email addresses were sent to residents in a huge data breach by the county council.
On Friday evening (June 8) West Sussex County Council sent out emails to people who commented on plans to build an incinerator in Horsham, the vast majority of whom objected.
People who received the message could see the email addresses of all the other recipients.
So far it has not been confirmed how many email addresses were shared but more than 1,160 objections were received by the council.
The messages were informing residents the incinerator plans were due to be discussed at a meeting next Tuesday.
The council has apologised for the breach.
A spokesperson said: “We would like to apologise to anyone affected by this situation. We take our responsibility to ensure personal data is processed fairly and lawfully very seriously, and we have written to everyone affected to advise them of the situation.
“In accordance with our data security breach management processes and in line with ICO guidance we took immediate steps to respond to the risks posed by the data breach and now we will be undertaking a corporate evaluation with a view to establishing whether there are any steps we can take to improve security and minimise risks in the future.”