Leaks part of Westminster life for decades
Leaks enabled Winston Churchill to warn in the 1930s of Britain's lack of preparations for the looming Second World War.
Former MP Tam Dalyell used them to embarrass Margaret Thatcher over the sinking of the Argentine cruiser, General Belgrano, during the Falklands War, and they even helped Gordon Brown demolish John Major's government in the 1990s.
Moles and whistleblowers have been part and parcel of Westminster life for decades, embarrassing governments and ministers from the Left and the Right.
That's why there is a sense of alarm - even anger - among MPs of all parties over the arrest and questioning of Damian Green, the Conservative immigration spokesman, by a posse of Scotland Yard detectives.
Leaks usually usually relate to correspondence between ministers, incriminating emails or even draft white papers - often information which Ministers would prefer was not made public.
Often information comes in unmarked, anonymous brown envelopes. On other occasions leakers resort to more cloak and dagger tactics. One MP who got hold of advance copies of a recent government defence review left copies in envelopes hidden around the Palace of Westminster and then tipped off journalists where to find them.
The photocopier and now the widespread use of computers and email have made the work of the leaker easier - though Whitehall uses a variety of methods, including putting codes into individual photocopiers, to try to track down the moles.
The longer a government has been in office, the more vulnerable it is to leaks from disgruntled or disillusioned officials - and the more paranoid ministers become about them.
The recent spate of leaks from the Home Office included a disclosure that the Home Secretary knew the Security Industry Authority had granted licences to 5,000 illegal workers, but decided not to publicise it, and a letter to the Prime Minister warning that a recession could lead to a rise in crime.
While they did not concern matters of vital national security, they were highly embarrassing to ministers as they provided the Tories with valuable ammunition to attack the Home Office on sensitive issues such as immigration.
Ministers also fear there is another well-placed "mole" in the Treasury who is feeding information to the shadow chancellor George Osborne.
Just over a year ago, Mr Osborne pre-empted Government plans for a big cut in inheritance tax, one of the reasons why Gordon Brown called off plans for a snap election. When Labour announced their own more modest proposals a few weeks later, it looked as if they were simply playing catch up.
This week's recession budget has been one of most leaked financial statements that MPs can recall. Details of the 2.5% cut in VAT and the proposal for a new 45p top rate of tax for those earning over £150,000 were leaked in advance.
The Treasury insists it did not want the details to get out before the formal announcement - and they suspected that the Tories got wind of their proposals. But it is also possible that talkative Labour ministers "in the loop" also let slip what was about to happen.
It is highly unlikely that Mr Green will be charged with any offence, unless there is evidence that he actively solicited the Home Office mole to purloin documents or paid for the information he received.
The heavy-handed police action - including detaining Mr Green for lengthy questioning - was more likely intended to frighten off MPs who might be tempted in future to have dealings with whistleblowers.
And Mr Green's arrest has enabled the police to seize his computers and boxes of papers from his home and offices in their search for evidence which can be used in court.
The MP admitted earlier today that he had many times made public information that the Government wanted to keep secret. But it was information that the public has a right to know and "in a democracy, opposition politicians have a duty to hold the Government to account".
The Tory leader David Cameron has given Mr Green his strong backing, questioning why counter-terrorism police were spending their time searching an MP's office, arresting him and holding him for nine hours.
Mr Cameron said he wondered what message it sent out about the state of democracy in Britain today.
Denis MacShane, Labour MP for Rotherham, suggested the police now believed that MPs were so reduced in public status "that they are fair game for over-excited officers to order dawn raids, arrests and searches of confidential files".
The leaks will go on. But other MPs may now be wondering whether they will be next to get that knock on the door.
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