Lewisham Council has changed the way it handles election counts after a blunder in Forest Hill.
Although the error did not affect the overall result - which echoed the borough-wide result of a Labour victory - local Lib Dems say the mistake "beggars belief".
According to the party, 1,708 votes were counted twice - around one in sixth of all votes cast.
Lib Dem candidate George Crozier said: "It beggars belief that a basic error on this scale could take place without the returning officer or any of his staff spotting it."
He added: "We are pressing the council to publish the actual number of votes cast too so that the people of Forest Hill have their votes accurately recorded."
A Lewisham Council spokesman said: "There were not more votes cast than ballot papers. This was simply an accounting error which meant that the number of votes cast was incorrectly recorded on the result sheet, though the result was unaffected. Changes are being made to the accounting system to prevent this in future.
"The correct number of votes cast has been published by the council. We have also briefed all candidates, agents and the Electoral Commission."