Two hundred and fifty thousand households were supposed to receive calls with a recorded message from party leader Nick Clegg.
But the Information Commissioner's Office (ICO) said the party had broken privacy rules and that they will face prosecution unless they stop.
Anyone who answered the call heard a recording of Mr Clegg telling them about his conference speech for 30 seconds before he asked them to press buttons on their telephones to select political topics and register their opinions on the LibDems' stance.
The party argued that the calls did not constitute marketing as they also asked people questions about their views on Liberal Democrat policies.
But the ICO ruled that the calls constituted "direct marketing" which is not allowed to be targeted at anyone who has not given prior consent.
They said they had received a number of complaints from those who received the calls
The Scottish National Party also complained to the IOC about the calls.
The commissioner took action against the SNP over its cold-calling campaign targeting 500,000 voters in 2005 after a formal Lib Dem complaint.
The Lib Dems have said they will not appeal against the ICO's decision.