A South Yorkshire council says there are "lessons to be learned" after a box of postal votes was missed out of last week's local election count.
The ballots – for the Royston ward of Barnsley Council – were not included in the overnight count on Thursday.
The council said the votes would not have changed the outcome. Labour held the ward, with Councillor Dave Webster winning the seat.
His vote would have doubled if the missed ballots had been factored in.
But Sarah Norman, Barnsley Council chief executive and returning officer for the election, said the error was unacceptable.
She said: "The postal votes have now been counted, and the figures have been recalculated.
"The result has not changed. Dave Webster from the Labour Party is elected for the Royston Ward.
"This discrepancy in the number of votes and the declaration of incorrect figures is not acceptable.
"We're very sorry that this has happened.
"We invited the candidates and their agents to the Metrodome….to explain the situation and express our sincere apologies for any distress that this has caused."
She said all paperwork and data would be handed to the Electoral Commission for candidates and agents to inspect.
"There are clear lessons to be learnt from this, and we'll make sure we implement these in future election counts," she added.
Labour retained overall control of Barnsley Council following last Thursday's vote.