Candidates in the local election were not told about the number of late postal votes until after they could challenge any results.
Some 1,423 postal vote packs were delivered late to Brighton and Hove City Council, arriving ten hours after polls had closed in the election in May.
In four wards there were enough late postal votes to potentially change the outcome.
Among them is Brunswick and Adelaide, where former deputy council leader Hannah Allbrooke lost her seat by only six votes.
However, candidates were not told about the postal vote situation until after the deadline for election petitions had passed.
A Freedom of Information request which published communication within the council revealed concern that the situation could fuel conspiracy theories.
A council communications employee, whose name was redacted, said: “I think the problem here, in PR terms at least, is that we didn’t let the opposition parties know about the late postal votes until after the 21 days were up.
“I think this is where the risk of conspiracy theories is most likely to take hold.
“I get the fact that there’s no legal requirement for the returning officer to publish the number of postal voting packs received after the close of poll, but 1,423 of them is a pretty eye-catching figure and it’s clear that the Greens are keen to make as much political capital out of this as they can.”
Another council staff member later said an election petition would require people to demonstrate the authority did not follow the law.
They said: “We did follow the law by not counting votes that were received after close of poll. Getting into 'could they have lodged a petition' is getting away from the central point which is that they did not arrive on time.”
Green group leader Councillor Steve Davis has called for an independent enquiry to determine what happened.
He said: “In the 2019 local election there were 19 postal votes that arrived within 24 hours after the poll closed. In the last general election, there were 23,389 such cases across the entire country.
“For Brighton and Hove to have had 1,423 late postal votes is unprecedented, unexplained and unacceptable.
“People submit their votes with a full expectation that providing they’ve followed the rules their vote will be counted. If an error in the delivery process meant that legitimate votes weren’t counted, Royal Mail has a lot to answer for.
“Despite new information coming to light there is still no explanation as to why councillors were not told sooner within the deadline courts have to act.
“What is also concerning is that Labour seem to be remaining disinterested in getting to the bottom of these fundamental questions of democracy.”
A Royal Mail investigation concluded that postal votes were “sorted, cleared and delivered appropriately”, with the council considering the matter “closed”.
A council spokesman said: “In the recent council elections, there were a total of 31,018 postal votes issued. Royal Mail delivered 1,423 of these postal ballot packs to us on the day after the elections.
“The law is crystal clear that these couldn’t be counted as they arrived after voting closed at 10pm the night before.
“There can, therefore, be no doubts about the validity of the election results.
“We let candidates and residents know about the late-arriving ballot packs at the same time as these could not be counted and thus made no difference to the election process.
“There are always a number of late-arriving postal ballot packs. The normal explanation for this is people posting them late.
“However, the number of late-arriving postal ballot packs this time was much higher than we would have expected so our returning officer asked Royal Mail to investigate this.
“Royal Mail has investigated and assured our returning officer that all the postal votes were sorted, cleared and delivered appropriately. We, therefore, consider this matter to be closed.
“All wording on postal voting packs is statutory and therefore cannot be changed.
“We always advise people that postal voting packs must be returned by close of poll. Electors can return their packs to polling stations or Hove Town Hall.”
A Royal Mail spokesman said: “At Royal Mail, we are immensely proud of the role we play in the democratic process.
“Our specialist election team meticulously plans every aspect of the elections delivery programme and works closely with Brighton and Hove City Council to ensure everything runs as smoothly as possible.
“We have carried out a thorough and detailed investigation which has involved looking at the entire pipeline of election mail and interviewing people at relevant sites.
“We have concluded that all election mail was sorted, cleared and delivered as it came into our network.
“We will work with the council to consider any changes that may be appropriate for future elections.”
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