IMPEACHMENT: a word to thunder. For centuries, ordinary crimes have been pursued through our criminal courts: theft, murder, treason.

But in the United Kingdom’s uncodified constitution, the high court of Parliament has always retained authority to punish wrongdoers. Erskine May, the parliamentary Bible for Westminster’s conventions, rules and regulations, describes impeachment as a measure reserved for “high crimes and misdemeanours, beyond the reach of the law or which no other authority in the state will prosecute.” In principle, “all persons whether peers or commoners may be impeached for any crime whatever”.

It is for the Commons – for MPs – to initiate impeachment proceedings. But it is the Lords, on the red leather benches, who must determine the accused’s guilt or innocence of the charge made against them.

In its history, the Lords have tried a few of these “extraordinary offenders”, from Warren Hastings in the late 18th century – the former governor-general of India, Hastings was accused and acquitted of pauchling public money – to Henry Dundas, 1st Viscount Melville, at the beginning of the 19th century.

The alleged offences of these men were various, from neglect of duty in public office to abuse of power, the oppression of public rights and the misapplication of public funds – often into their own pockets. Dundas, whose statue still presides proudly over St Andrew Square in Edinburgh, was the last man to face impeachment before the House of Lords in 1806.

William Pitt the Younger’s right hand man, Dundas was sometimes characterised as the “uncrowned King of Scotland”. Some historical scholars have described his “despotism” as retaining a tight grip over Scotland’s often rotten parliamentary burghs, dispensing favour and position, and punishing those who strayed too far from the party line or who were tempted by radical ideals of parliamentary reform.

Perhaps his most famous political victim was Thomas Muir of Huntershill, a democratic reformer who distributed copies of Thomas Paine’s Rights of Man to the weavers of Dunbartonshire. A marked man because of his reforming sentiments, Dundas and his extended family of placemen and allies accused Muir of sedition for spreading Paine’s democratic gospel. Convicted after a rough and biased assize, the reformer was transported across the sea by Lord Braxfield and his brother judges, many of whom expressed regret they couldn’t have him gruesomely executed.

But Dundas subsequently found himself under parliamentary scrutiny for having his hands in the till, and misappropriating public funds. After speeches and accusations, evidence and hearings, their lordships voted to acquit Dundas.

For more than 200 years, the House of Lords have never again been called upon to exercise their ancient right to try malefactors accused of high crimes and misdemeanours by the Commons.

So can Parliament still launch impeachment proceedings against those whose crimes will otherwise go unpunished? Is Mr Blair vulnerable to prosecution, his failings having been exposed so comprehensively by Sir John Chilcot and his colleagues?

Views differ. Erskine May regards impeachment as “obsolete”. But as a matter of law, the procedure has never been abolished. In principle, it ought still to be available. You have to wonder, however, how much credibility any modern impeachment could really command. A full criminal trial, launched by MPs, heard and adjudicated by hundreds of ancient, unelected peers? The political sitting of a “noble” jury – a good chunk of whom are political placemen and women?

Impeachment is a procedure from another century, covered in stoor. It sits uncomfortably with modern principles of democracy and accountability. It may not be compatible with fundamental rights, including the idea that people accused of crimes must be tried by an independent and impartial tribunal.

But there it sits in the parliamentary repertoire – still, neglected, mute – awaiting revival.