Investigation launched after watchdog accidentally copies all who complained about Sun article in email telling them their complaint would not be explored

The information commissioner has launched an investigation after the personal details of nearly 500 people who had complained about an article in the Sun were exposed by the press watchdog.

The complainants were all copied into an email from the Independent Press Standards Organisation (Ipso) informing them that their complaints would not be taken forward. Each was able to see the others’ addresses because they were placed in the “cc” field, rather than the “bcc” field. In many cases, the email addresses also revealed the complainants names.

“This is shocking, given we complained in confidence. Some of us are now receiving unpleasant messages as the email distribution list seems to have been leaked,” one of the complainants, who wished to remain anonymous, told the Guardian.

All of the people affected had complained about a Sun article in which the columnist Kelvin Mackenzie said Fatima Manji, a Muslim journalist who wears the hijab, should not have been allowed to report on the massacre in Nice because it was suspected to have been an Islamist terror attack.

The group was told that only someone directly affected by the article could make a complaint about anything other than its bare factual accuracy. They were told that, since they were neither Fatima Manji, nor acting on her behalf, their complaints could not be investigated.

Embarrassingly for the watchdog, Manji herself also received the email. Ipso later clarified that it would be investigating her complaint and has “internally addressed the issue of how we communicate with complainants by email”.

Jim Killock, executive director of Open Rights Group said: “This is a basic error that could have negative consequences for the people who complained, and may also deter others from making complaints to Ipso in the future.”

“Organisations have a legal obligation to protect our data and they should take this responsibility seriously.”

Manji has been defended by her employer, which called the comments “offensive, completely unacceptable and, arguably, tantamount to inciting religious and even racial hatred”.

On Friday, the Information Commissioner’s Office said: “We’re aware of a potential incident and are making enquiries.”