People heard a recording of Lib Dem leader Nick Clegg
The Liberal Democrats broke privacy rules with their use of automated phone calls featuring leader Nick Clegg, the Information Commissioner's Office says.
The party have been told to stop using them or face possible prosecution.
The Lib Dems, who made 250,000 of the calls last week, have been issued with an enforcement notice after complaints to the commissioner's office.
Senior Lib Dems had insisted the calls would not break any rules as they were for "genuine market research purposes".
Lib Dem chief executive Lord Rennard told the BBC last week the aim of the calls in the wake of Mr Clegg's party conference speech was to "guide" the party to the issues worrying voters in 50 key seats.
An automated 30 second voice message from Mr Clegg was played out during the early evening calls, with recipients tapping numbers on their handsets to respond to questions about education, health, tax, crime, environmental and economic policies.
Ahead of the calls being made the Scottish National Party, reprimanded in the past after a Lib Dem complaint about their use of unsolicited automated calls using the voice of actor Sir Sean Connery, complained to the Information Commissioner.
The Information Commissioner's Office has now ruled that the Lib Dem calls constituted "direct marketing", which are not allowed unless someone has given prior consent.
In a statement the ICO said: "After reviewing the phone script, the ICO concluded that the calls were made for the purpose of promoting the Liberal Democrats and therefore constituted direct marketing."
It went on: "In addition, the ICO has now received a number of complaints from those who received calls without giving their prior consent.
"The Privacy and Electronic Communication Regulations forbid the use of automated unsolicited direct marketing calls to any individual who has not previously given their consent to receive such calls."
Deputy Information Commissioner David Smith said: "The ICO 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.
"Many people find unsolicited automated calls particularly intrusive and annoying so it is important that any organisation making such calls ensures that individuals have given their consent before they are targeted."
The enforcement notice gives the Liberal Democrats 30 days to stop using the calls and any breach would be a criminal offence.
The SNP's MSP Tricia Marwick criticised the Information Commissioner's Office's failure to act to stop the calls "given that they had 12 hours notice" of the Lib Dems' intention to make the calls.