Barrow council has apologised after packs containing incorrect electoral information were sent out to some residents in the borough.
Documents instructed postal voters to select one candidate in Westmorland and Furness Council wards where more than one person could be chosen.
The borough council says letters have been sent directly to those affected.
But Hazel Edwards, who is aiming to be elected in the Hawcoat and Newbarns ward, feared the amendment may have come too late for some.
“By then, I imagine, many people would have returned their postal vote forms,” she said. “I think, if people want to carry it to its extreme, there could be a legal challenge.
“I’m not personally in favour of a legal challenge, but it’s a major mistake.”
Ms Edwards, who serves as a borough councillor in the Hawcoat ward, said she had been contacted about the issue by nine people. She did not know how many residents had been in touch with fellow councillors, but expressed concerns about the potential scale of the error.
A spokesman for Barrow council insisted the print issue was only present in a limited number of packs sent out to households in the borough.
“The instruction sheet incorrectly asked voters to select one candidate per ward, although the ballot paper correctly shows the number of candidates they can vote for in their ward,” said the spokesman.
“Action has already been taken to clarify this issue in line with advice from the Electoral Commission – with letters sent directly to those affected.
“We would emphasise that not all postal voters received this incorrect information, but we apologise for any inconvenience it may have caused.”
The unitary authority elections on May 5 will see 65 councillors elected to the area currently covered by Barrow Borough Council, South Lakeland District Council and Eden District Council.
Those voted in will serve one year on a shadow authority that will oversee planning and preparation for the new council, which is due to begin operating in April 2023.
