The by-election takes place on Thursday
The Information Commissioner is launching an investigation after the Conservatives accidentally sent details of 8,000 people to a radio station.
The e-mail sent contained the names, addresses, telephone numbers and intentions of voters in the Crewe and Nantwich by-election.
Deputy Information Commissioner David Smith said the disclosure, to Manx Radio, raised "serious concern".
The Conservatives admitted a "mistake" had been made.
The information, which had been gathered by the Tories during the campaign for Thursday's by-election, was contained in three Excel files, and was sent in an e-mail to Manx Radio by accident, potentially breaching data protection laws.
The e-mail was received by the radio station at lunchtime on Wednesday.
It sent it on to the local newspaper in Crewe, although it is understood that both emails have now been deleted.
Mr Smith said: "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 to establish the full facts and see where responsibility lies."
Following the revelation that the email had been sent to the media, Cabinet Office Minister Ed Miliband wrote to Conservative leader David Cameron.
In his letter he said: "I would hope you are treating this as a serious matter and you are contacting the people concerned and apologising for this breach of their personal privacy. I'm sure they would wish this to happen before the polls close tomorrow [on Thursday]."
A Conservative spokesman said: "The email shouldn't have been sent but we have done all that we can to ensure that it remains confidential.
"The email - which was based on information from the electoral register - was sent in error to a journalist.
"Within two and half hours the recipient was informed. Both he and the local newspaper he sent it on to have now given undertakings that the information on the email has been destroyed.
"A third media outlet was also forwarded the email. It is unclear by whom. They too have given an undertaking that the contents will not be published.
"An internal investigation has already discovered that it was due to human error but an internal inquiry has begun to ensure that it does not happen again."
Polling stations in Crewe and Nantwich open at 0700 BST and close at 2200 BST on Thursday. The result is expected at about 0230 BST on Friday.
The by-election follows the death of long-serving MP Gwyneth Dunwoody last month.