In English law, hearsay evidence is generally inadmissible in court. The "rule against hearsay" means that if a person who actually saw or heard something relevant, or knew something relevant, can no longer tell the court about it - because he is dead or untraceable - then someone to whom he communicated that evidence cannot relay it to a court.

This is because the court would not be able to test the credibility or reliability of the original source of the evidence through examination and cross-examination. There are however, several exceptions to this rule and one of those is that the declaration of a dying person can be used in law as evidence.

Andrei Nekrasov, Mr Litvinenko’s friend, says that before Mr Litvinenko died, he told him "the bastards got me but they won’t get everybody". Aware that he was set to die, Mr Litvinenko dictated a final message that stated:

"As I lie here I can distinctly hear the beating of wings of the angel of death. I may be able to give him the slip but I have to say my legs do not run as fast as I would like.

"I think, therefore, that this may be the time to say one or two things to the person responsible for my present condition. You may succeed in silencing me but that silence comes at a price. You have shown yourself to be as barbaric and ruthless as your most hostile critics have claimed...you may succeed in silencing one man but the howl of protest from around the world will reverberate, Mr Putin, in your ears for the rest of your life.

"May God forgive you for what you have done, not only to me but to beloved Russia and its people."

At a murder trial, an oral or written statement by the victim about the cause of his injuries is admissible as evidence provided that at the time it was made, the victim was under a settled hopeless expectation of death and would have been competent as a witness.

The rationale for allowing such evidence was explained in a judgement in 1789 by Chief Baron Eyre. He said that the commending feature of such statements is that they are:

"declarations made in extremity...when every hope of this world is gone; when every motive to falsehood is silenced and the mind is induced by the most powerful considerations to speak the truth."

This reasoning is not entirely sound. Some dying people might be weak of mind or want to incriminate an enemy. But the law has taken the view that it is better to allow the evidence in court and assess it against such possible drawbacks than simply to exclude it altogether.

Gary Slapper is a Professor of Law at the Open University and a Times Online columnist