Staff shortages have been identified as one of the reasons why thousands of votes were not counted in Plymouth in the general election.
More than 6,000 votes were missed out of the result of one constituency, an independent investigator has found.
Some postal voters did not receive any ballot papers at all - 87 failed to arrive at registered addresses.
Plymouth City Council said a "dedicated team" had been set up following the mistakes.
Investigator Dr David Smith's report says:
- Thirty five thousand postal vote electors received two polling cards - one for their postal vote and one for the polling station.
- Three hundred and thirty one electors who received poll cards were later removed from the register and some were turned away from polling stations.
- Nearly 2,000 registered postal voters did not receive their packs.
- Of these, 1,839 were issued with a replacement postal vote pack, leaving 87 where delivery was not possible.
- There were 6,587 votes not included in the result for Labour MP Luke Pollard, although he would still have won comfortably over Conservative Oliver Colvile.
It cost the Conservative-controlled authority £68,000 to solve the issues, as they had to pay for additional staffing and couriers to get the replacement voting packs out.
Mr Pollard tweeted: "Catalogue of errors from @plymouthcc in #ge2017 was totally unacceptable and incompetent. Process to restore trust in council must begin now."
The report blamed a "long-standing problem with insufficient capacity" on election staffing.
It also blamed election staff "capability", which was made worse by the retirement of a longstanding electoral services manager.
The recruitment process was not complete when the snap election was called in April 2017.
"As a result, the registration and electoral system and process lacked integrity, robustness and adequate quality control and assurance," said the report.
Council leader, Conservative Ian Bowyer, said it was "important for all councillors that both they and our voters have complete confidence in the council's electoral service".
He said: "I am totally committed to ensuring that this happens."