Originally Posted by padser
illegal: prohibited by law or by official or accepted rules;
So from an English language point of view, any tort [at the very least any tort that is prohibited by statute] is illegal.
If the question you are asking is whether they are criminal, then no - they are civil offences
I agree with Padser (and thereby disagree with some of the other opinions on this page). Illegal in the strictest sense means against the rules. One common example is in the game of chess: if you are in check (but not checkmate) and you make a move that doesn't take your king out of check, this is commonly called an "illegal" move.
However, since your question relates to whether or not it is correct to say, in a forensic atmosphere, that tortuous activity is illegal, I believe it is.
For the purposes of what can be defined as illegal, it does not matter whether you refer to criminal law, civil law, constitutional law or company law, nor does it matter whether you refer to statute law or common law. Any breach of any law can be termed illegal, although I do accept that the most common connotation of illegal is criminal.
Most people would term a breach of the criminal law as illegal - murder is illegal. As an example in tort, in false imprisonment cases it is often pleaded that the defendant falsely and illegally imprisoned the plaintiff (especially when one is suing An Garda Siochana). An inquiry under Art 40.1 of the constitution is an inquiry into the legality or illegality of the detention of a person (the illegality here being the breach of their constitutional rights without lawful authority), and in company law, if a company acts ultra vires, it can accurately be described as acting illegally.
However, for completeness I should say that the terms illegal and unlawful are often considered to have different meanings. While this is a point more of style than of substance, illegal is often used to suggest a proven breach of the law, unlawful to be an assertion of illegality or a synonym for without legal authority. These terms are often used in criminal law, i.e. unlawful use of a computer (S9 Theft and Fraud, 2001) or the former (although technically never valid) unlawful carnal knowledge under S1 of the 1935 Offences against the Person act. Similarly, S2 of the Non Fatal Offences Act, 1997, includes "without lawful excuse" in the definition of assault.
There are several torts which involve illegality; detinue (illegal possession of another’s property), trespass to land (entering or being on another’s land without lawful authority), and I think that most people would accept that where an act is both tortuous and criminal it could definitely be termed illegal.
Therefore, it depends on your own idiomatic preferences. It is perfectly correct to call tortuous activity illegal. In my own vernacular, defamation is (from strongest and most formal to weakest and most casual): illegal, unlawful, tortuous, wrong, against the rules...
People often say that something is not legal, but not exactly illegal either, and while this would suggest that there is a grey area in between legal and illegal, I think that this applies to what people consider to be wrong but at the same time does not violate any positive law. A defamatory statement breaches tort law, and therefore is illegal.