As law makers, MPs are used to the cut and thrust of debate. But the courts offer a more incisive style of interrogation, as Martyn Jones, MP for Clywd South, found recently.

Mr Jones sued a newspaper for libel after it published a story saying he swore at a guard in Parliament. The cross-examination he underwent was every bit as robust as question-and-answer contests in the House of Commons.

Cross-examination is the process in which one side’s lawyer questions the other side’s witness. The aim is to weaken the opponent’s case, expose any unreliability in their witnesses and to establish facts favourable to the cross-examiner’s own case.

Unlike the knockabout routines in the legislature – and contrary to the court room scenes you will have seen on TV – cross-examination in court is a sharp and searching dialogue. It can be done politely and without hostility. As Sir John Mortimer, QC, recalled his late father – also a distinguished barrister – saying, “the art of cross-examination is not the art of examining crossly”.

The procedure has a long history. The noun “cross-examination” was first recorded in a case in 1729, although the technique itself is much older, appearing in one case involving a will in Norwich in about 1200. It is an excellent method of clarifying the facts of a disputed matter. It is a serious intellectual contest fought in the threat of grave consequences.

Here’s how it works. In a criminal case, after a prosecuting advocate has finished questioning (or “examining”) a witness called to support the Crown’s case, defence counsel is given an opportunity to examine (or “cross-examine”) that witness in an attempt to poke holes in their evidence. Later, the prosecution is similarly given an opportunity to examine the defence witnesses. In a civil case, both claimant and defendant, or their advocates at least, are given the opportunity to cross-examine each other and their respective witnesses.

A common tactic is for counsel to lead a witness through a series of propositions he will have to agree with until he is cornered into agreeing with one final, deadly point. At other times, advocates will pivot quickly to a riveting question, catching the witness by surprise. In opening the cross-examination of Frederick Seddon, who was on trial for the murder of his lodger Miss Eliza Barrow, Sir Rufus Isaacs, the Attorney-General, began:

ISAACS: Miss Barrow lived with you from July 26, 1910, to September 14, 1911?

SEDDON: Yes

ISAACS: Did you like her?

This flummoxed Seddon and he didn’t regain his composure. He could see that if he said he had liked her he would be asked why he had put her in a pauper’s grave, but if he said he hadn’t liked her he would tilt the case further against himself. Decidedly, a killer question. Seddon was eventually executed for the murder.

The advocate has many advantages over the witness, such as knowing the rules of evidence and choosing the line of inquiry. That doesn’t mean the advocate always gets the upper hand. A barrister in Ireland once began a cross-examination of an Irish Prelate with the words: “Am I wrong in thinking you are the most influential man, and decidedly the most influential Prelate or Potentate, in the Province of Connaught?” The witness replied: “Well, you know, they say these things, but it is in the sense that they would say that you are the very light of the Bar of Ireland: these are children’s compliments.”

One masterful cross-examination was that in 1909 by Sir Edward Carson, KC (King’s Counsel), of the witness William Cadbury, director of the chocolate company. Between 1901 and 1908, Cadbury obtained half its cocoa from islands off the coast of Angola, exploiting forced slave labour. Cadbury knew about the slavery and profited hugely from it for years but didn’t reveal it to the public. Instead, it traded on its reputation as a model employer. Meanwhile, people were snatched as slaves and forced to march up to 1,000 miles to the plantations, and killed if they didn’t keep up. When the Evening Standard published an article criticising the company’s labour practices, it sued, claiming it made them look like “a bunch of canting hypocrites”. After brilliantly cross-examining William Cadbury for five hours, peppering him with questions about how much human suffering was involved in the production of the chocolate, Carson concluded by asking him, “Have you formed any estimate of the number of slaves who lost their lives in preparing your cocoa from 1901 to 1908?” It was a bit like asking, “Have you stopped beating your wife?” – a question that condemns the person either way. The director replied meekly: “No, no, no.” The jury found the company had been libelled, but awarded damages of only one farthing.

Sir Henry Curtis-Bennett, KC (1879-1936), famously said that an advocate cross-examining a witness should never ask a question if he didn’t already know the answer. A modern case in point was recounted by Sir Oliver Popplewell in 2003. As a young barrister defending a man charged with careless driving, Mr Popplewell was cross-examining a prosecution witness who had testified that the defendant had been speeding. The witness was repeatedly pressed to estimate the speed of the car but declined. Having satisfactorily established the witness’s incompetence in car-speed estimation, Mr Popplewell didn’t sit down but asked one final fatal question: “Why are you telling the court you cannot estimate the speed of my client’s car?” The witness’s response was calm and clear: “Because I have never seen a car go as fast as that in all my life!”

Times Recommends

• BGC 'set aside £40m to lure Tullett brokers'
• Legal aid review to ensure value
• Fears as 21 magistrates' courts face closure