Labour breached privacy rules by making unsolicited automated phone calls to almost half a million people without their consent, the Information Commissioner's Office ruled today.
The calls, featuring a recorded message from Coronation Street actress Liz Dawn, were targeted at around 495,000 people in areas with strong Labour support to encourage them to turn out to vote in the local and European elections last June.
Deputy Information Commissioner David Smith said calls of this type can cause "annoyance and disruption" to those receiving them.
He ordered Labour to ensure that no further automated direct marketing calls are made without consent, warning that failure to comply would be a criminal offence which could lead to prosecution.
Labour had previously said it would stop automated calls of this kind after the Commissioner received a complaint about the use of the Liz Dawn recording in 2007, said Mr Smith.
At the time of the 2009 elections, the Commissioner received further complaints from a member of the public and the Scottish National Party.
Labour confirmed that it had made the calls to approximately 495,000 recipients, in what were believed to be Labour supporting areas, and that the majority of numbers were obtained using commercially-purchased lists.
Mr Smith said: "The Information Commissioner's Office has consistently made clear that the promotion of a political party counts as marketing. We have previously issued detailed guidance to all major political parties on this subject.
"The Labour Party has breached privacy rules by making automated marketing calls to individuals who have not consented to receiving such calls.
"The fact that the calls were targeted at what were believed to be Labour supporting areas confirmed our view that they were designed to promote the Labour Party's electoral cause by encouraging Labour supporters to vote.
"Automated calls can cause annoyance and disruption, which is why it is so important for organisations making such calls to gain the consent of individuals."
The Labour Party has 28 days to appeal against the enforcement notice under the Privacy and Electronic Communications Regulations (PECR), which were introduced by the Labour Government in 2003.
A Labour Party spokesman said: "The Labour Party considered advice from the Information Commissioner's Office prior to making these calls and believed that we had abided by this advice.
"We will now examine this decision in detail. The Labour Party has always sought to abide by the Information Commission guidelines and will continue to do so in the future."
Last year's complaint was lodged with the Commissioner by Scottish National Party MSP Joe Fitzpatrick.
He today said that concerns were first raised over Labour's use of automated calls in 2005, but no enforcement notice was issued until today. Notices have been issued against the Conservatives and SNP in 2005 and the Liberal Democrats in 2008.
Mr Fitzpatrick said: "I welcome this belated action to protect voters in their homes but there are real questions to be answered over how the Commissioner has applied the rules.
"The Information Commissioner must explain why it has taken him five years to act over dodgy phone calls from Labour but he has not applied the same restraint to other political parties.
"The SNP sought guarantees from the Information Commission in 2005 that all parties would be treated as the SNP were treated. However it seems while the SNP, Conservatives and Liberal Democrats were served enforcement notices immediately Labour has been allowed an ill-fated second chance.
"The Information Commissioner must also explain why these calls were not stopped immediately rather than allowing half a million people to be bothered by a desperate Labour party."