Nick Clegg could block the coalition's plans to expand internet snooping agenda if he is advised it is "illiberal", he has pledged.
The Liberal Democrat leader was grilled on the proposals contained in the draft justice and security bill by party delegates during a question and answer at the party conference in Brighton.
Civil liberties campaigners have reacted with outrage at the government's plans, contained in the Queen's Speech, to give law enforcers wider scope to monitor details about private phone calls.
Under current legislation police and security services can establish when a person has made a telephone call, to whom the call was made and for how long it lasted.
The coalition wants to legislation to extend this power to other forms of online communications, like Skype.
"Some people say you can't distinguish between the how, where and when and the content. That seems to me to be a technical issue hugely important," the deputy prime minister said.
"If you start giving the authorities the powers to look at the content under the guise of how where when, that's far more illiberal."
Lib Dem MP Julian Huppert, a member of the Commons' home affairs committee, will advise Clegg on whether the proposals are acceptable to the party.
Clegg added: "If the questions and queries and dilemmas posed by the joint committee are not satisfactorily answered by the Home Office, of course it won't become a bill or an Act. We are taking this pre-legislative scrutiny very seriously indeed."
Lib Dems will express their collective concern at another of the bill's proposals in a policy motion to be debated on Tuesday.
Delegates are set to resist plans to give ministers the power to seek that civil hearings and trials can take place in secret where national security is judged to be at risk of being damaged.