The Liberal Democrats would "kill" plans for more monitoring of emails and internet use if they were not watered down, the party's president has said.
Tim Farron told BBC 1's Andrew Marr Show he was prepared to look at draft legislation when it is published but would not back "authoritarian" laws.
The plans would allow government listening post GCHQ to monitor internet traffic in real time.
The Home Office has said the move is key to tackling crime and terrorism.
Under the proposed Bill, internet firms would be required to give intelligence agency GCHQ real-time access to communications on demand.
Although intelligence officers would not be able to access the content of emails, calls or messages without a warrant, it would allow them to identify who an individual was in contact with, how often and for how long.
They would also be able to see which websites someone had visited.
The government has faced criticism over the plans, announced last week.
Some senior Conservatives joined Lib Dems and civil rights campaigners to warn the proposals would cause an intrusion into freedom and privacy.
Mr Farron told the BBC: "I am prepared to recognise that there is obviously a need in modern society with new technology to have a look at what needs to be given to the security services, but only if it is absolutely clear there is no universal access.
Free society 'threat'
"But we are prepared to kill [the plans], be absolutely clear about that, if it comes down to it.
"If we think this is a threat to a free and liberal society then there would be no question of unpicking them or compromising, this just simply must not happen."
The Lib Dem president said he would be "surprised" if the Bill ended up looking "anything like the press reports we have had this week".
He said he felt that a government that includes Liberal Democrats "should ensure that Britain ends up a more liberal place, not less".
Attempts by the last Labour government to bring in monitoring of internet communications failed after opposition from MPs, including Conservatives.