The ship of state has always leaked. Gordon Brown was open that he had received information from a mole in the Treasury in the mid 1990s, as had Winston Churchill about lack of preparation for war in the 1930s, and sometimes it is in the public interest that information should leak. On the other hand, the proper functioning of the civil service and the normal processes of the state are also in the public interest, and departments simply cannot run properly if confidential advice to ministers is leaked every other day. There must be a minimum level of trust between ministers and civil servants.
In the news at the moment is the arrest and bailing of Damian Green MP, apparently "on suspicion of conspiring to commit misconduct in a public office". The facts about Mr Green and the leaks concerned are entirely unclear at the moment. This commentary only explores the legal background to events as they may unfold.
Those leaking in the 1990s and afterwards no doubt felt somewhat safer in this enterprise than beforehand, by the fact that the Official Secrets Act 1989 had replaced the former notorious section 2 of the Official Secrets Act 1911 (long title of the 1989 Act: "An Act to replace section 2 of the Official Secrets Act 1911 by provisions protecting more limited classes of official information"). The idea was to protect whistle-blowers up to a point - the line being drawn where the leak might be damaging within the terms of the 1989 Act. (The object was also to remove a public interest defence in the 1911 Act which worked against the prosecution in the Clive Ponting case).
In order for official information to be protected under the 1989 Act as an official secret, normally it has to fall into one of the categories that could give rise to an offence under that Act, that is to say security or intelligence information, defence information, information concerning international relations, foreign confidences, information which might lead to the commission of crime, and information obtained under the special investigation powers in the Interception of Communications Act 1985 and the Security Service Act 1989. In addition the disclosure normally has to cause harm to the UK or its interests as set out in the Act for the category in question, or (broadly) it could reasonably be believed that harm could occur.
If a civil servant were improperly to pass other (unprotected) information to, say, a journalist or member of Parliament, that is not to say that the civil servant concerned would not have breached the terms of his or her appointment as a civil servant and be liable to disciplinary action and dismissal (he or she would be likely to be dismissed and might suffer other civil sanctions), but that is a civil rather than a criminal matter.
Step in however the ever mysterious common law, and conspiracy. There is an ancient common law offence of "misconduct in a public office". For an offence of misconduct in a public office to arise, a person holding a public office such as a civil servant or police officer must, without reasonable excuse, wilfully do (or fail to do) an act in the execution of the office which is of such a serious kind as to amount to a breach of the trust imposed in the office holder.
At common law there is also the crime of conspiracy to commit an offence, and in statute of aiding and abetting or counselling and procuring an offence. This can make people who are sometimes called "accessories" also guilty of an offence.
So although, say, a civil servant who leaks unprotected information might escape under the 1989 Act, he or she might still be guilty of the common law offence of misconduct in a public office (punishable by up to life imprisonment incidentally).
If a particular journalist or member of Parliament were to enter into a scheme or arrangement with an official in advance, under which he or she would knowingly be passed unauthorised information from a department by the official, the journalist or member may in certain circumstances also be guilty of conspiracy in, or aiding and abetting, the misconduct of the official in public office, even if the information is not protected under the 1989 Act. This forms part of the background to secrecy law, but I must make the point that there is no indication at all that anything like this has happened in the case of Mr Green. Even if it had, it might well be innocuous.
There is also an interesting point in the case of a member of Parliament, about whether the office of member itself comprises an "office" covered by the common law offence, in which case an MP, subject to article 9 of the Bill of Rights 1689, might be liable as a principal rather than just an accessory. This is highly theoretical though.
Those charged with such offences have one thing on their side: it is unlikely a jury would convict except in serious cases genuinely affecting the security of the state. That is not to say that any charge or trial might not be politically mbarrassing (for the government as well as the person subject to the charge).