Police have been urged to start a murder inquiry into Dr David Kelly’s death following further allegations that he did not commit suicide.

Officers have been told the government scientist could not have taken an overdose of painkillers.

This overdose was found by the original pathologist to be one of the causes of his death.

Dr Andrew Watt, an experienced clinical pharmacologist, says he has told Thames Valley Police it is not possible Dr Kelly could have swallowed more than a ‘safe’ dose of two coproxamol tablets because there was so little in his system after death.

He said: ‘I reported to the Thames force that I believe that the death of Dr Kelly may have been murder. I have received an acknowledgement and they have given me an incident number.

‘I have been told that the inquiry is being conducted by a very senior officer.’

A second development also casts doubt on the suicide verdict of the Hutton inquiry – which took the place of a formal inquest.

The Mail has established that Dr Kelly left an upbeat answerphone message to his friend Nigel Cox just days before his body was found on July 18, 2003. Dr Kelly said he was looking forward to joining him for a game of cards on July 23.

Mr Cox, who played in the same pub cribbage team as Dr Kelly, was on holiday at the time and only received the message after the scientist’s death. 

The message said ‘Hi Nige, I will see you at crib next Wednesday’, said Mr Cox, adding that the tone convinced him his friend was not suicidal.

He contacted Thames Valley Police and suggested they listen to it because he believed it was an important indication of Dr Kelly’s state of mind and showed he had plans for the future. According to Mr Cox, police expressed interest in the message but it was never collected by investigating officers.

Dr Michael Powers QC, representing a group of doctors who have begun legal action to secure a full inquest on Dr Kelly, said: ‘Establishing evidence of intent to commit suicide is essential.

‘It is very surprising Thames Valley Police failed to follow up this important lead. Were there to be a coroner’s inquest this evidence would have to be investigated.’ 

Last night Thames Valley Police declined to comment on why they did not collect the answerphone message. A spokesman said: ‘There has been a thorough investigation into the death of Dr David Kelly and that investigation has been the subject of a major inquiry by Lord Hutton.’

Police confirmed they had received Dr Watt’s allegation that the scientist was murdered.

‘This will be considered by senior officers at the next meeting we have on David Kelly. We have not yet decided what to do about Dr Watt’s information,’ said a spokesman. 

Dr Kelly’s body was found in woods near his Oxfordshire home shortly after he was unmasked as the source of a BBC report claiming the government ‘sexed up’ a dossier on Iraq’s weapons. 

Attorney General Dominic Grieve is facing mounting demands for an inquest and has promised to make a decision on this shortly.