Those involved in the exchanges included Julie Dore, leader of the council, Gillian Duckworth, director of legal and democratic services, councillor Bryan Lodge, then cabinet member for environment, John Mothersole, then chief executive and Paul Billington, then director of environment.
The emails related to the planning of Operation Quito, the name given to the policing work around tree felling which was part of a £2.2 billion highways improvement scheme.
Staff members wrongly put ‘legally privileged - not subject to FOI’ in the subject header of the messages in an attempt to prevent them being released through requests made under the Freedom of Information (FOI) Act.
The names of most of the staff members who attempted to use this disclaimer were redacted from FOIs, but the requests revealed multiple were sent by Mr Billington.
Out of the dozens of emails that used the subject header it included one that revealed there was a target for felling trees - which the council previously denied having - and another that was about a press release.
Councillor Douglas Johnson, leader of the Green Party, said: “It’s a really clumsy attempt to withhold information. It makes it obvious there is something they want to hide, if it genuinely wasn’t subject to FOI they wouldn’t need to do that. It shows a culture of dishonesty and it’s actually pretty nasty.”
It was revealed when Marcus Combie, street tree activist, only received some of the emails he asked for under the FOIA which prompted him to ask for an internal review - which was pushed back three times before completion.
The internal review confirmed there were 41 emails missing from the response because they were from the account of 'someone who had left'.
More importantly, Mr Combie said it highlighted that in just one month from one account there were 20 emails which used the ‘not for FOI’ subject header.
A follow up FOI from fellow street tree activist Justin Buxton uncovered a message sent from someone on behalf of the FOI department telling chiefs what they were doing was wrong: "[The emails] were not legally privileged and we cannot declare that something is not subject to FOI."
Mr Combie has now requested that all emails from that account which relate to street trees using that subject header are released.
He has also called for an investigation into the extent of the practice across the council and how it may have affected information being released.
A spokesperson for the Information Commissioner said an investigation would need to be undertaken to determine if what officers and councillors did was illegal and stated: “Public authorities which fall under the FOIA must comply with the law and refer to our Code of Practice. Anyone who is not satisfied with the response they receive can request an internal review. If they are still not satisfied, or have received no response at all, they can make a complaint to the ICO."
Sheffield Council were contacted for comment.