The terror law watchdog has warned that new anti-extremism measures planned by the Government could be "very dangerous".
David Anderson QC, the independent reviewer of terrorism legislation, has expressed serious concerns about the Counter Extremism Bill.
Mr Anderson said people who are "miles away" from terrorism could be investigated because of their religion under expected proposals to combat those with vocal or active opposition to fundamental British values.
"The difficulty, I think, with the Extremism Bill - which we haven't seen yet, it hasn't actually appeared, although it's twice been announced - is that they want to bring in an element of coercion to this process, and they say that if you are going to engage in extremist activity, then you could be made subject to some coercive measure whereby you might not be able to use social media, or you might be limited in your associations, or in where you can go, and so on.
"I think silence coerced by law, as an American judge once put it, is a very dangerous business, particularly when you are looking at something as vague as extremism.
"I've not met anyone who can really define it in a satisfactory way.
"There's talk of British values, people who oppose diversity, democracy, the rule of law, tolerance. We can all think of people, I'm sure, we would love to see silenced in one way or the other, preachers of hate, of whatever kind.
"I think the trouble with rules like that is that all sorts of people are going, in principle, to be subject to them. People are going to complain about neighbours, they're going to complain about people they work with, the police are going to feel they have to investigate all sorts of people who are miles away from being terrorists, but may just practise religion in a conservative way, or may have eccentric political views.
"That's not really the way we have ever done things in this country. We didn't ban communism during the Cold War, and I think we should be strong and robust enough to argue back.
"To start applying the force of the law to people who oppose certain values, I think, is a really difficult and dangerous line to go down," he told BBC Radio Four's World At One.
Asked if such provisions in the Bill could themselves be opposed to fundamental British values, Mr Anderson, who is to stand down from his review of terror laws role in February, said: "That's of course one way of putting it."