Read how the Times covered the London assassination of Bulgarian dissident Georgi Markov in 1978

On New Year’s Day in 1988, Abdullah Ali, an Iraqi businessman who had been living in London for eight years, joined three compatriots for dinner at a restaurant called Cleopatra in Notting Hill.

The next morning, he was taken ill with flu-like symptoms and was admitted to hospital. There his condition rapidly deteriorated — his hair fell out, he developed excruciating skin and joint pain, and paralysis and respiratory failure began to set in.

Fifteen days later he was dead — but not before he had begun to wonder whether something had been added to his vodka. He was right: the Westminster Coroner recorded the cause of his death as bronchiopneumonia due to thallium poisoning.

Abdullah Ali is thought to have been a victim of Saddam Hussein’s secret service, which used thallium sulphate as its poison of choice. Detectives believe he was an undercover agent who became disillusioned and was murdered before he could defect.

Hundreds of Iraqi dissidents met their end in similar fashion. Thallium’s slow action enabled the poisoners to adopt a particularly sinister tactic: dissidents would be released from prison, and even allowed to emigrate, but not before their food or drink had been laced with a fatal dose. Sometimes, it was administered during a “reconciliatory” drink with the prisoner’s former guards.

In his recent book, The Elements of Murder, the chemist John Emsley recounts the case of Majidi Jehad, who was given orange juice at a Baghdad police station while collecting the passport he needed to travel to Britain. He died of thallium poisoning when he arrived at his destination.

Salwa Bahrani, a Shia activist, was killed with yoghurt that had been laced with thallium. In 1992 two army officers, Abdallah Abdellatif and Abdel al-Masdiwi, escaped to Damascus where they fell ill. They were flown to Britain, where they were treated successfully.

France also used the poison to kill a guerrilla leader in Cameroon in 1960, and the United States is suspected of using thallium in one of its many attempts to kill President Castro of Cuba. Sidney Gottlieb, a CIA chemist, conceived a plan to poison the Cuban leader by putting thallium powder in his shoes. This method would have caused his hair to fall out, robbing him of his iconic beard, as it destroys hair follicles.

Thallium was used medically and cosmetically before its lethal effects became known. Though the fatal dose for an adult is 800 milligrams, or less than a quarter of a teaspoonful, 500 milligrams would be prescribed to treat ringworm. Thallium depilatory creams were popular in the 1930s.

Thallium is a metal that is usually administered as a poison in the form of one of its salts, typically thallium sulphate. Its toxicity derives mainly from the fact that charged thallium atoms are almost exactly the same size as potassium ions, which are critical to many bodily functions. It essentially mimics the action of potassium, replacing working ions with inert ones that cripple the nervous system.

Early symptoms are similar to flu and gastroenteritis. This is followed by extreme band-like pain around the body, particularly in the feet and joints. The cause of death is usually heart or respiratory failure, as the nervous system collapses.

It is an attractive tool for murder because it is soluble in water, colourless and virtually tasteless and odourless. A fatal dose can also be given in one go and the body is not good at excreting the toxin by itself. The most effective antidote is potassium ferric ferrocyanide, a chemical better known as the dye Prussian blue.

The toxin has also been used by many murderers, of whom the best known was Graham Young, “the St Albans Poisoner”. In 1962 Young, then 15, used a selection of poisons to kill his stepmother and attempt to kill other members of his family.

He was committed to Broadmoor but released in 1971 and found a job in a photographic studio in Bovingdon, Hertfordshire. Soon afterwards, his foreman, Bob Egle, 59, fell ill and died. Another colleague, Fred Biggs, 60, then died with similar symptoms, and other became unwell.

Young was arrested after asking the company doctor whether he had considered thallium poisoning as a possible cause of the mystery illness. Thallium was found at his flat, along with a diary in which he had noted the doses given to his workmates, together with their effects. He was given four life sentences in 1972.

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