The admission came from the local authority’s head of highways, Paul Billington, as he gave evidence at the High Court where protesters face injunctions over the felling of trees.
Legal action is being taken over continued protests about thousands of trees being being chopped down as part of the council’s Streets Ahead contract with Amey.
During cross examination, John Cooper, QC, the barrister representing protesters, asked Mr Billington if he was aware that Amey had been convicted of corporate manslaughter at the time the contract was signed in 2012..
He asked: “Were you aware before you contracted with Amey that they had been convicted of corporate manslaughter?”
Mr Billington replied: “No, I certainly wasn’t. My understanding on review is that the council weren’t either.”
Mr Cooper said Amey received the conviction in September 2008.
He asked Mr Billington: “This was the first corporate manslaughter conviction there was because a pit collapsed, killing one of their workmen. They did not tell you about that did they?”
The officer replied: “I have given you my answer.”
The barrister continued: “The contract should have been completely re-negotiated shouldn’t it?, after you found out that Amey had been convicted of corporate manslaughter. This contract is illegal isn’t it?”
Mr Billington replied: “No, it’s not.” He added that officers at Sheffield City Council were comfortable with the decision after taking legal advice.
Mr Cooper then asked: “That’s the way it’s done in Sheffield isn’t it? What the officers say - the council does.”
Mr Billington replied: “I don’t know where you are taking this.”
Mr Cooper also asked: “It is Amey pulling the strings in all of this with the council, isn’t it?”
The officer replied: “I can hear what you are saying, Mr Cooper, and I utterly and totally reject it.”
The case is currently on the second day of a hearing which is expected to finish tomorrow.
The council is seeking permanent injunctions against the defendants and ‘persons unknown’ at the trial.
It says campaigners are protesting inside safety barriers ‘unlawfully’ and as a result holding up work - which is costing the taxpayer money.
If the campaigners are served with injunctions and break them they will be in contempt of court, and could face fines or even prison.
The defendants claim the council have put profit first when making the decision to chop down thousands of healthy trees without exploring alternative solutions.
They also say the case highlights the burden the local authority had been placed under by entering into a PFI contract with Amey.