Conservative campaign workers for tonight's Crewe and Nantwich by-election has lost the personal information of 8,575 voters, throwing the party's campaign into disarray.
The data protection watchdog is set to investigate the incident after the names, addresses, voting intentions, financial status and phone numbers of the voters was made public.
The incident occurred after campaign workers sent out an email with the data to an overseas journalist.
Labour are seizing on the mistake as evidence of the Tory party's inability to govern competently.
"This shows the Conservatives should not be allowed to run a bath, let alone run the country," a party spokesman said.
A Tory spokesman said: "An email was sent in error to the wrong person. We discovered immediately what had happened and have received written confirmation from the original recipient that the information had been destroyed."
The deputy information commissioner, David Smith, confirmed an investigation into the incident has been launched.
"It is a serious concern if people's personal details and voting intentions have got into the public domain. Voting information is particularly sensitive. We will be launching an investigation," he told the Mirror newspaper.
It remains to be seen how far the incident will revive Labour's fortunes in tonight crucial vote. Analysts suspect it may be too late for the news to affect people's voting intentions, but the mistake has added an element of uncertainty into what seemed like a guaranteed Conservative win tonight.