I'm aware that criminal and illegal aren't the same thing but in both the US and the UK copyright infringement can be a criminal matter (these things aren't mutually exclusive - things aren't either or, many are both).
Under the Copyright, Designs and Patents Act 1988 (http://www.legislation.gov.uk/ukpga/1988/48/contents) copyright law in UK allows for criminal prosecution where there is deliberate intent or passing-off in a commercial context. Section 107 (http://www.legislation.gov.uk/ukpga/1988/48/section/107) outlines both the detail of the criminal liability (and it is expressly criminal liability) and the punishments that can result, including prison and fines.
The reason it's more commonly treated as a civil matter is that for the injured party to recover damages they'd need to pursue a civil case as well as the criminal case so they don't normally bother.
Someone else has pasted a link supporting that this is the same in the USA and, as it was a condition of the WTOs Agreement on Trade Related Aspects of Intellectual Property Rights (TRIPS) that laws be passed making copyright infringement a criminal offence, this is almost certainly true for any signatory (153 member states including the UK, USA, India, most (if not all) of Western Europe and anyone who is anyone except, for some reason, Russia.
Under the Copyright, Designs and Patents Act 1988 (http://www.legislation.gov.uk/ukpga/1988/48/contents) copyright law in UK allows for criminal prosecution where there is deliberate intent or passing-off in a commercial context. Section 107 (http://www.legislation.gov.uk/ukpga/1988/48/section/107) outlines both the detail of the criminal liability (and it is expressly criminal liability) and the punishments that can result, including prison and fines.
The reason it's more commonly treated as a civil matter is that for the injured party to recover damages they'd need to pursue a civil case as well as the criminal case so they don't normally bother.
Someone else has pasted a link supporting that this is the same in the USA and, as it was a condition of the WTOs Agreement on Trade Related Aspects of Intellectual Property Rights (TRIPS) that laws be passed making copyright infringement a criminal offence, this is almost certainly true for any signatory (153 member states including the UK, USA, India, most (if not all) of Western Europe and anyone who is anyone except, for some reason, Russia.