Nick Clegg was today facing embarrassment after the Information Commissioner expressed "concern" over plans to bombard 250,000 voters with automated phone calls featuring a recording of the Liberal Democrat leader.
The Commissioner said the party had failed to provide a script of the call and had not given details of the target audience.
Handing over such information would allow a decision on whether the exercise was for "market research purposes or for the purpose of promoting the Liberal Democrats".
Once further details were provided the Commissioner would "take whatever action is appropriate".
The intervention follows the plan to target households in 50 marginal seats after Mr Clegg's first conference speech as leader.
The US-style cold-calling technique, due to be carried out this evening, will see anyone answering their phone hear Mr Clegg tell them about his conference speech today to activists in Bournemouth.
It will last for 30 seconds and ask them to press buttons on their key pad to select political topics and register their opinions.
The party believes it is the first time such a technique has been used by a British political party.
Today the party's chief executive Lord Rennard insisted the calls would be used for market research purposes and denied what the party was doing was against the law.
He told BBC Radio 4's The World at One: "We are doing something quite different...what we are doing is asking people for their opinions on issues and parties are entitled to ask people for their opinions on issues for genuine market research purposes."
He added: "We want to hear from people...Nick Clegg and the Liberal Democrats are a people and a party who do want to listen to what people think."
A statement released by the Information Commissioner's Office said: "The Information Commissioner's Office is concerned to hear about the Liberal Democrats' proposed use of automated phone calls.
"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.
"The ICO has consistently made clear that the promotion of a political party counts as marketing and we therefore expect all parties to take into account the rules on unsolicited marketing when making such calls.
"However it is not a breach of the regulations to use automated calls to carry out market research to individuals.
"The ICO has previously taken enforcement action against the Scottish National Party for breaching the regulations on marketing and our action was upheld by the Information Tribunal.
"We have also produced detailed guidance notes on the application of the regulations for political parties in this area
"This morning, we have contacted the Liberal Democrats to clarify the nature of the calls and to establish who they are contacting, but we have not yet been shown the proposed script.
"Once we have considered the script and the target audience we will be able to establish whether in our view the calls are for market research purposes or for the purpose of promoting the Liberal Democrats.
"We will then take whatever action is appropriate."