A council that wrongly warned thousands of voters they would not be able to vote in the general election has been criticised for the blunder.
Swindon Borough Council admitted nearly 3,000 voters were contacted in error.
Residents were told in the letter they would be "removed from the Register of Electors" as they were "not entitled to remain registered" at their property.
The council has asked people to ignore the letter as it was sent "by mistake".
Originally it was thought the letters had been sent to 1,500 voters but the council has since said about 3,000 people were contacted.
Jennifer Miles said she had a "borderline heart attack" when her letter arrived at her home, adding: "I was absolutely aghast."
"It's these kind of things that make people nervous or disillusioned about going out to vote, and that's negative for society," she said.
"Swindon is a marginal constituency and the vote really, really does matter here."
By Dan O'Brien, BBC Wiltshire political reporter
This is a cock-up rather than a conspiracy, but the timing is pretty dreadful.
About 3,000 people were sent a letter by mistake suggesting they may not be able to vote, in a town which historically is known to be a close race at election time.
The fact councils send out letters like this isn't unusual, they all regularly check up on who is registered to vote at each address to reduce the risk of voter fraud.
But the error here, we're told, is who those letters got sent to. The council's printers used the wrong distribution list - if you have such a letter and it's dated 4 November, ignore it. You will get another very soon explaining what's happened, and you will still get your polling card.
South Swindon was particularly close last time around - in a constituency of 70,000, the Tories won a majority of just 2,400.
So it's easy to see why political campaigners across the spectrum are worried and clearly angry about anything that could stop local people voting on 12 December.
The letter, dated 4 November, said the Conservative-run council was reviewing its "registration entitlement" and it was the authority's "opinion" that the resident was "not entitled to remain registered at this property".
Recipients were warned that if they did not appeal against the decision within 14 days they would be removed from the electoral register.
Another person who received the letter, Anthony Clarkson, said: "I couldn't believe what I was reading. I thought it must have been some sort of scam."
Andrew Whitford, another recipient, said he thought it "could affect the outcome of the general election and should be investigated completely".
South Swindon Labour parliamentary candidate Sarah Church said: "Any hint of removing anyone's right to vote is simply unacceptable and a mistake on this scale is really unforgivable."
Ms Church is due to stand against Conservative Robert Buckland, the Brexit Party's Justin Stares, Liberal Democrat Stan Pajak and Steve Thompson for The Green Party.
Mr Buckland called it an "unacceptable error" and Mr Pajak described it as a "threat to democracy".
In a statement, the council said anyone who received a letter should ignore it.
"The error occurred after our printers used an incorrect distribution list," it said.
The authority added it would be writing to every resident who had received a letter in error to explain what had happened and to apologise.