Voters in a London borough have lost their chance to take part in the election despite registering before the April deadline.
Hackney Council has told the BBC about 100 people will be unable to vote.
Claire Mason, a Hackney resident, said she was told the system probably could not process the number registering to vote in the week up to 20 April.
A council worker told her up to 20,000 people registered that week.
"He reiterated that I had to face the possibility that I could not vote, which is outrageous," she said.
Ms Mason, who has lived in the east London borough for 12 years, said she and her partner were among those who had registered just before 20 April.
"Maybe if I hadn't registered so close to the deadline I might have had a better chance," she said.
"This election more than ever we're being told numbers are tight," she added. "And with all kinds of random parties and candidates now in the frame I feel that my vote counts more."
Another Hackney resident Steph Wilson wrote on Instagram: "I've just been told I can't vote due to the rubbish new voting registration forms that Hackney Council failed to send to me. Shame on you @hackneycouncil."
'No need for polling card'
The borough has two parliamentary constituencies - Hackney North & Stoke Newington and Hackney South & Shoreditch.
Hackney Council said about 100 people were thought to be affected and issued a statement which said enabling people to vote was an "absolute priority".
"Due to issues caused by the new Individual Electoral Registration system we did not receive some applications until late into the process, as details were cross checked by the Department for Work and Pensions, via the government's new online registration service."
It said voters did not need a polling card to vote, but just needed to give their name and address to staff at their polling station.
An Electoral Commission spokesperson said it was aware of the issues in Hackney, which it understood related to specific problems with the council's IT system.
He said: "Any voter that has concerns about whether they are registered should contact their electoral services team in Hackney to check this and, if needed, to find out where their polling station is."
Candidates in Hackney North & Stoke Newington:
Labour - Diane Abbott
Liberal Democrat - Simon de Deney
Green - Heather Finlay
UKIP - Keith Fraser
Conservative - Amy Gray
Animal Welfare Party - Jon Homan
Communist League - Jonathan Silberman
Candidates in Hackney South & Shoreditch:
Christian Peoples Alliance - Taiwo Adewuyi
Cannabis is Safer than Alcohol - Paul Birch
TUSC - Brian Debus
Green - Charlotte George
Independent - Russell Higgs
Labour - Meg Hillier
Liberal Democrat - Ben Mathis
Workers Revolutionary Party - Bill Rogers
Campaign - Gordon Shrigley
UKIP - Angus Small
Conservative - Jack Tinley