“IT IS one of those irregular verbs. I give confidential security briefings, you leak, he has been charged under section 2a of the Official Secrets Act.” That was how the mandarins of the 1970s Whitehall TV comedy Yes Minister summed up the process by which information enters the public domain.

With the arrival of e-mail, text messaging and blog sites, the technology has moved on, but the principles remain the same. When is it a briefing, a leak or a smear? Sometimes the answer is clear; in other cases, it can be all three.

Ministers or their aides routinely contact journalists to brief them confidentially on their plans. The Commons Speaker periodically moans about parliament being bypassed, but the process is a well-established part of Westminster life. Journalists get an exclusive story and the politicians get a chance to float their policy ideas.

Less commonly, a briefing may be accompanied by the distribution of a confidential document. This is what is known in the trade as a “licensed leak”. The information may hitherto have been private, but ultimately nobody’s reputation has suffered in the process.

Another kind of leak is where an official – or even on occasion a minister – puts out a document calculated to damage a rival minister.

A leak inquiry may be ordered by the permanent secretary of the relevant department, but when the negative briefing is “red on red” (that is, Labour on Labour) such investigations are almost always inconclusive.

As Damian Green, the Tory immigration spokesman, discovered, leak inquiries are taken seriously only if the damaging information falls into the hands of opposition spokesmen.

An unlicensed leak from a government source can be accompanied by a whispered personal attack on a rival minister. This is known as a smear. Some smears are straightforwardly untrue, such as the suggestion on a website that James Purnell, the work and pensions secretary who is engaged to be married, was gay. Others seek to undermine a politician by targeting his family. Cherie Blair, for example, became a proxy target for Labour enemies of her husband.

The most damaging smears are the ones that carry a grain of truth. When the New Statesman quoted an unnamed No 10 source as saying David Miliband could never be Labour leader because he is “too grand", it would have struck a chord with many MPs.

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