The hair and eyebrows are gone, the face is sallow, the gaze is empty; all classic signs of poisoning.

John Henry, a clinical toxicologist, who examined Mr Litvinenko, said he was "quite seriously sick" and there was no doubt he had been poisoned by thallium.

"It is tasteless, colourless, and odourless," he said. "It takes about a gram - a large pinch of salt in your food - to kill you."

The substance was used in rat poison and insecticide but was phased out because of fears it could cause cancer. It attacks the nervous system and internal organs and causes peripheral neuropathy, which starts as a tingling and leads to loss of sensation, causing victims to feel as if they are wearing gloves or socks.

Further damage leads to pain, muscle weakness and changes in the skin, hair and nails. Early symptoms of thallium poisoning are similar to flu with headaches and muscle aches.

But within three days symptoms may worsen with vomiting, convulsions, dementia and coma.

The advantage to a poisoner trying to use the substance as a murder weapon is that the severe symptoms are delayed and can be confused with other neurological conditions such as Guillain Barre syndrome.

Graham Young, known as the Teacup Poisoner, used thallium to kill his stepmother when he was 14, in 1962. He was released after nine years and went on to poison two workmates with thallium in tea. He died of an apparent heart attack in Parkhurst jail in 1990.

In Japan this year, a 17-year-old girl was sent to reform school after poisoning her mother with thallium.

A graphic account of thallium poisoning is given in Agatha Christie's 1952 novel The Pale Horse.