Islington Council has made yet another data gaffe – by leaking personal information of 559 ex-employees to its own former leader.

Terry Stacy, the Liberal Democrat who led the council between 2008 and 2010, said he was “gobsmacked” at the town hall’s “unbelievable sloppiness”.

Mr Stacy made a Freedom of Information request to the council asking how much money has been spent on redundancies since 2010. Of the 947 staff who were made redundant, 559 were named – including the confidential sum they were paid.

Islington has a well-documented history of data breaches. In 2015, personal information about penalty charge notices – including medical and criminal records – was freely available on the council’s parking appeals website.

And in 2012, officers inadvertently handed details of 51 Andover Estate residents in Finsbury Park to the same gang members they had ­reported to the council for terrorising their neighbourhood.

After flagging the error on Friday, Mr Stacy received an urgently worded email from the council saying the document “must be deleted”.

Mr Stacy, who obliged, told the Gazette: “I was somewhat gobsmacked that the council has done this – yet again. This is another major breach of confidentiality.

“Staff would have been given major training after the other breaches. It shows unbelievable sloppiness. The council seems to have total disregard to confidential information.

“There needs to be a root and branch review. I’m just thankful they didn’t release it to somebody else, as in the wrong hands it could have been unbelievably damaging to the hundreds of former council staff.”

An Islington Council spokesperson admitted: “Unfortunately a mistake was made in response to a Freedom of Information request. Regrettably, as well as disclosing the information we are required to give, we also revealed the surnames and employee numbers of 559 individuals who have left the council’s employment since 2012. This is personal information which we did not intend to reveal.

“We are very sorry we made this mistake. We are taking action to notify all those affected – all of whom are former council employees.

“We are treating this matter seriously, and are grateful to Mr Stacy for alerting us to our mistake. We are notifying the Information Commissioner’s Office (ICO) and reviewing our procedures.”

In all, Islington paid £12.2million making 947 staff redundant since 2010. The spokesperson added: “This is a large sum but it represents one-off payments in the context of £170m of ongoing savings.”