Nick Clegg is clashing with the Prime Minister over reviewing privacy laws in Britain, in light of the terrorist attack on the French magazine Charlie Hebdo last week.

Both David Cameron and Andrew Parker, the head of MI5, have called for Britain's intelligence services to be given new powers to read the contents of citizens' communications, in order to avoid similar "mass casualty attacks" here.

This signals the return of the so-called Snooper's Charter to the political agenda. The Communications Data Bill, which would give the government new powers to see the contents of any of our messages, phonecalls and other communications without a warrant, will go in the Conservative manifesto for the election this year. Parker referred specifically to the supposed danger of our privacy being "so absolute and sacrosanct", and supports more powers against privacy for the security service.

However, Nick Clegg is reaffirming the Lib Dems' opposition to this legislation this week. One of his party's often-trumpeted triumphs in office is having blocked the Tories' Bill, and it appears Clegg is as determined as ever to oppose what he sees as a threat to our civil liberties.

He told the BBC's Today programme this morning that while he supports plans to, "strengthen our defences to make ourselves physically safe", he sees a law closing in on our privacy as a "slippery slope . . . if we start to censor or self-censor ourselves as a society."

His argument is that the Bill is an "unworkable proposition", because much of our communication, and industries that we rely on, are based outside of the UK's jurisdiction anyway. He also questions the efficiency and necessity of monitoring everyone's messages: "Scooping up vast amounts of information on everyone, children, grandmas, who go on garden centre websites . . . Every single individual in this country, people who would never dream of doing us harm."

Clegg also put himself on the side of civil liberties by defending our right to be, "free to offend each other in an open society", when asked about the illustrations of Charlie Hebdo cartoonists. "We have laws which do inhibit people from inciting violence and hatred, but we protect the right for people to say things which other people might not like."

In a speech later today, the Deputy Prime Minister is expected to say:

The irony appears to be lost on some politicians who say in one breath that they will defend freedom of expression and then, in the next, advocate a huge encroachment on the freedom of all British citizens. Let me be really clear, we have every right to invade the privacy of terrorists and those we think want to do us harm – but we should not equate that with invading the privacy of every single person in the UK. They are not the same thing. The Snoopers' Charter is not targeted. It's not proportionate. It's not harmless. It would be a new and dramatic shift in the relationship between the state and the individual.

The Snooper's Charter became a symbol of the "red lines" that the Lib Dems have been able to draw in office, and therefore their ability to temper the Tories' worst excesses by being in coalition. This is how the Lib Dems are attempting to appeal to the electorate ahead of May – a heart for the Tories, and a spine for Labour. Perhaps this fresh privacy debate will do Clegg a favour in reminding the public of his ability to counter the ruling party.