A murder trial jury has been told that a retired doctor's decision not to give evidence in his trial should not point to his guilt.

Howard Martin's barrister today defended the former GP's decision not to go into the witness box at Teesside Crown Court.

Anthony Aldridge, QC, conceded that Dr Martin's version of events had not been tested under cross-examination from prosecutor Robert Smith, QC, but asked the jury to try to understand his position.

Mr Arlidge said many people would be reluctant to be questioned in such difficult circumstances, and Dr Martin had found it tough enough sitting in the dock as expert witnesses in the case criticised his methods.

On day 24 of the trial, he told the jury of six men and six women: "You have seen my learned friend and I cross-examining people.

"I speak for myself only, and I am not a very nice person. If I am cross-examining somebody, I probe as hard as I can on behalf of my client. My learned friend is no pussy-cat either.

"Dr Martin has had to sit there and hear himself being called totally incompetent . . . by all these grand fellows, who have come here from their ivory towers with their portfolio careers."

Mr Arlidge added: "You can imagine the sort of gruelling time he would have had in the witness box.

"You can understand why a 71-year-old man with his life on the line would not wish to expose himself to that."

The former County Durham GP, who now lives in Gwynned, north Wales, denies murdering three of his patients, Frank Moss, 59, Stanley Weldon, 74, and Harry Gittins, also 74.

Mr Moss, who had lung cancer, was visited at his home in Eldon, near Bishop Auckland, on March 13, 2003, and was given huge injections of morphine even though the first one left him unconscious, and died the following day.

Five days later, Stanley Weldon, from nearby Coundon Grange, was given a similar dose of morphine when Dr Martin visited him at his nursing home to treat him for pneumonia, and died the next day.

Mr Gittins, from Newton Aycliffe, was given a cocktail of injections of morphine, diamorphine and chlorpronazine by Dr Martin on January 21 last year, and died hours later.

But Mr Arlidge claimed the prosecution had failed to sufficiently prove the drugs killed the three men, that Dr Martin knew exactly what effect they would have on his patients, and that he did not administer the medication to relieve their pain.

The barrister told the jury that Dr Martin's evidence was contained in written statements he gave to police when he was arrested after the death of Mr Gittins.

He said: "Whatever you do, don't say 'Well, he has not given evidence, he must be guilty'. It is transparently clear that that is not the case."

Earlier, Mr Smith concluded his closing speech by telling the jury that Dr Martin saw the three men as "hopeless cases on their way out" whose treatment would be a waste of time and money.

He said: "He decided to put them to sleep as quickly as possible and in doing that he stepped outside the bounds of the doctor's duty in a civilised society, and he exercised a power which neither his profession nor the law allows him to exercise."

Mr Arlidge is expected to finish his speech tomorrow and the judge is expected to start summing up.

The case continues.