A council published documents in the middle of the night to "avoid scrutiny" over its controversial tree-felling programme, a watchdog found.
Sheffield City Council has been involved in a long-running dispute over the scheme, which saw thousands of trees removed.
A report by the local government ombudsman said it acted with a "lack of transparency, openness and, on occasion, honesty".
The council has apologised in response.
As part of the £2.2bn, 25-year street improvement works, contractors Amey removed trees that were assessed as dangerous, dead, diseased or dying.
But many residents felt trees were removed unnecessarily, and it provoked scenes with protesters, police and arrests.
The report also found the council drew a "veil of secrecy" over expert advice, and resorted to "subterfuge" in posting false tree removal notices.
A council response to recommendations from the Independent Tree Panel (ITP), established in a bid to resolve disagreements, was published at 04:30 on the day it scheduled contractors to remove trees.
"It says it did this because it believed protest groups had software which alerted them to the council publishing its responses to the ITP group online", the report said.
'Restore people's faith'
It follows a complaint in November 2016 about tree-removal on Rustlings Road in Sheffield.
Contractors Amey started felling trees at 05:00, removing eight despite specialists and the independent tree panel saying only one needed removing.
The man who initially complained has now died and the ombudsman said the council must apologise privately to his family, as well as publicly for the way the tree saga was handled.
Councillor Mark Jones, member for the Environment, Street Scene and Climate Change, said the council "fully accepts" the report findings.
He said: "Whilst this report is reflective of a very different and difficult time, we are continuing to make real and significant progress towards a more transparent and collaborative future when it comes to managing our valuable street tree stock."
The council has developed a new tree strategy and the report said the council has been "working hard to restore people's faith".
But it warned that councils must be "honest, open and transparent" or risk losing trust and faith.