The work to replace the cladding on a Sheffield tower block which failed a fire safety test could cost the council around £1m.
The Star understands local authority chiefs are drawing up a price plan which expects to run into seven-figures for work on Hanover tower in Broomhall.
And now Sheffield Council's housing chief has promised a 'thorough' investigation into why a different cladding was used on the tower which failed the test.
Janet Sharpe, the top council officer in charge of housing admitted the council 'don't know' why a different agreed cladding was installed on Hanover tower block but said she was 'determined' to find out.
In a statement issued to the press in the wake of the Grenfell tragedy in London, the council said 'all of our cladding is fire proof'.
The news from Hanover sparked fresh fears from residents living there.
Speaking to The Star at Sheffield Town Hall, Ms Sharpe said: "We don't know how that happened and that's what we've got to look at. The cladding change could've happened for all sorts of different reasons.
"My job now is to look at the information to understand why's that happened and now we'll be talking to the contractors themselves.
"People move jobs, records were in different places so my job is to get all the information and reach an understanding with the contractor."
The work was done as part of an 'extensive homes programme' in which the council approved over 39,000 homes around 2009.
When asked why the work was signed off without the agreed type of cladding, Ms Sharpe said: "There is a lot of checks in place in terms of our management of contracts and this particular contract, the contractor was responsible for designing the work so it's managed different to other contracts.
"There are checks and balances in place as part of any refurbishment programme. But sitting here, I don't know the reasons for the change and I need to know why this has happened.
"In most of our tower blocks we have rain screen cladding system. Our blocks including Hanover are fire proof. But this is the outer leaf that encloses the insulation and at the time when we said that, we hadn't received the new information that led us to have this sample taken tested.
"But even that aside, all the insulation is fire retardant, the way the system works around cladding and we've done extensive fire checks in the property. So if the question is, are our tower blocks fire proofed then the answer is yes but we've got one element that we need to remove and replace."
All 24 tower blocks in Sheffield have now been tested and the results of those are expected next week.
Council leader Julie Dore said money would have to be pulled from other council budgets if the Government didn't stump up the cash.
But housing boss Sharpe said: "Regardless of where we get the money from, we will take this cladding down and we will replace it.
"We're in constant contact with residents and we're doing all we can to reassure them."