Sheffield council has apologised after a year-long independent investigation found it “deliberately” misled residents over the city’s street tree-felling programme.

A resident, Sally Goldsmith, questioned the decision to remove an alder tree in Aldam Way, Totley, and was told by a council contractor in August 2017 that removal was recommended because of “decay within the stem”. It added that an independent surveyor had “confirmed a decay pocket … as well as damage to the highway network”. A similar email was sent to another complainant.

However, Goldsmith subsequently used freedom of information laws to get hold of a copy of the surveyor’s report. She found the report had actually recommended the tree be retained, that decay was healing well and that pavement damage was “minor”.

Her complaint has been upheld. A soon-to-be officially published ruling by the Local Government and Social Care Ombudsman has made a finding of fault against the council.

The ombudsman ruled that the contractor, Amey, had “deliberately set out not to reveal the true advice it had received”.

Goldsmith said the case raises concerns about the council’s approach to the wider felling programme. Campaigners believe many trees have been removed unnecessarily.

She said: “I’m also pleased they raised real concerns over the confusing information, the veracity of the council statement that ‘felling is always the last resort’ and their lack of transparency when dealing with the public. This reflects on the council, not only with regard to this tree but for many threatened trees throughout the city.”

The report said the council had accepted the findings while noting the authority had said it was under “exceptional pressure” due to public protests over its wider tree-felling policy, which involved the removal of thousands of street trees and their replacements with saplings.

The tree in question had been recommended for removal in 2015 as part of its Streets Ahead highways project.

However, the council rejected advice from its own surveyor that could save the tree, maintaining it was damaging the pavement. Two attempts were made to fell the tree but were frustrated by protesters.

The ombudsman said one of the council’s faults in the case was failing to give the independent panel all of its reasons for wanting to fell the tree, meaning it did not consider the issue of possible highway damage.

The report, which refers to Sally Goldsmith as “Ms C”, said: “We find the council at fault for not giving its independent tree panel all its reasons for wanting to remove this tree and for a misleading reply to an enquiry from Ms C.

“We consider the faults have caused uncertainty about whether the tree needs removing. The council has agreed to reconsider its decision as part of its new strategy towards street trees and to apologise to Ms C for misleading her.”

The official strategy for Streets Ahead – under which about 5,500 trees have been removed since 2012 and replaced with saplings – said that felling was only considered as a “last resort”. But the ombudsman said the evidence in this case to support this claim was “far from convincing”.

The council has agreed to provide written apologies to residents and said it would retain the tree “if possible”.

A Sheffield council spokesperson said: “We will undertake the agreed actions within the timescales outlined in the ombudsman report and consider whether any lessons can be learnt as a result.”

Earlier this month seven campaigners who were wrongfully detained while protesting against tree felling in the city were awarded a £24,300 payout by South Yorkshire police.

The seven protesters were arrested between November 2016 and February 2017 and detained for up to nine hours under an obscure trade union law that was incorrectly used, the police watchdog found last year.