Complaints about Labour's use of automated phone calls are being looked into by the information commissioner.
Automated calls are allowed, as long as they are not marketing calls
The method - dubbed "robocalling" in the US - greets people with a recorded message asking them to press a number to indicate their preferred party.
Calls had been made in recent weeks, as speculation rose about a snap election - now ruled out by Gordon Brown.
The Lib Dems say the calls are illegal and want the commissioner to be more "assertive" in investigating them.
The Labour Party told the BBC it did not comment on its polling methods.
But a spokeswoman for the information commissioner told the BBC News website that, in the last few weeks: "We have had some complaints from individuals regarding the Labour Party's use of automated calls which we are now looking into, really to establish whether they are marketing calls or not."
Under an EC directive - unsolicited calls made solely to gather information are legal, but are banned if they are used to promote a product - or political party.
BBC journalist Huw Jones, who lives in west London's Ealing North constituency, is among those who has received a "robocall".
He said: "'I picked up the phone and I heard a female voice saying "This is a recorded message from the Labour Party'. It then asked me who I would vote for in a general election, press one if you would vote Labour, two for Conservative etc.
"I held on hoping to get a real person so I could ask then to remove my number from their records and not to phone me again but it just repeated so I put the phone down."
The information commissioner's office looked into complaints about the Labour Party's use of automated calls before the 2005 general election.
But no action was taken because it said it did not have "sufficient evidence they were marketing calls".
However the Scottish National Party was rapped for unsolicited automated calls which used the voice of actor Sir Sean Connery to urge people to vote SNP.
Election ruled out
The Lib Dems argue that unsolicited "information gathering" calls in which parties ask people if they would mind being sent more information, do contravene the EC directive.
Asked about the Lib Dems' use of automated calls, spokesman Mark Pack told the BBC: "It's not something we do. Our view is that it's illegal."
He said the information commissioner should be more assertive against Labour: "It is clear that they are making automated phone calls to people where they have not had specific prior consent from those individuals.
"I think when they (the information commissioner's office) have pursued this issue before they have been far too willing to take at face value the answer given to them."
The Conservatives say they prefer to rely on their associations around Britain to make the calls and in 2005 used call centres in London and the North of England to do so, but a spokesman could not confirm or deny that the party had used automated phone calls in its polling.
Prime Minister Gordon Brown has now confirmed that he will not call an early election this autumn.