Liberal Democrat president Tim Farron today vowed the party would "kill" proposals for increased monitoring of emails and internet use if they were not watered down.
Mr Farron said he was "prepared" to look at draft legislation, dubbed a "snoopers charter", when it is published but warned he was "in no mood" to back "authoritarian" laws.
Many Lib Dems were "horrified" by the plans, which would allow Government listening post GCHQ to monitor internet traffic - times, dates, numbers and addresses - in real time.
Mr Farron told BBC 1's Andrew Marr Show:
The Government has faced an intense backlash over the plans, with senior Conservatives joining Lib Dems and civil rights campaigners in warning they would cause a gross intrusion into freedom and privacy.
ITV News political correspondent Libby Weiner describes the potential clash as "the next faultline in the coalition government."
Mr Farron said he would be "surprised" if the Bill ended up looking "anything like the press reports we have had this week".
He said: "My conversations with the Deputy Prime Minister and others reveal that there has been significant movement this week."
Mr Farron added:
Nick Clegg insisted earlier this week the Government was simply "updating the rules" to allow the police and security services "to go after terrorists and serious criminals" who are using the internet to communicate.