A widow is calling for a change in the way death certificates are completed after a doctor incorrectly guessed the cause of her husband's death.

As Sue Willis struggled to come to terms with husband John's sudden death, she found herself embroiled in a battle for an accurate death certificate.

Mr Willis, 65, was referred to Yeovil District Hospital after falling ill last Easter. It was thought he could have been suffering lymphoma, but medical experts ruled this out.

Two weeks later, the grandfather-of-three suffered renal failure and was transferred to Bristol's Southmead Hospital, where he later died.

Despite never receiving a conclusive diagnosis of her husband's condition when he was alive, the doctor filling in the death certificate declared the cause of death as a heart and bowel condition. This was accompanied by the words "best guess".

"In our case, their 'best guess' turned out to be completely inaccurate," said Mrs Willis. "These untruths on his death certificate not only causes us immense distress but leaves us and other families open to inaccurate medical histories and skews UK health statistics."

Determined to discover what killed her fit and healthy husband, Mrs Willis demanded a post-mortem examination.

She said: "Having lost my husband to a mystery illness I immediately requested a postmortem in the search for answers. To my surprise I was immediately issued with a death certificate despite the fact that the cause of death had not been established."

The family was divided as one half wanted to pursue the truth and the other half simply wanted to bury their father. This resulted in two ceremonies, one being a memorial service and the other a full funeral.

The post-mortem examination revealed that Mr Willis died from a rare form of Large B Cell Lymphoma – not, as stated on his death certificate, a heart and bowel condition.

Mrs Willis, who lives in Pilton, near Glastonbury, said: "Had we not asked for a postmortem, John's children and grandchildren would assume a history of a heart condition and would be completely unaware of this hereditary disease. The diagnoses have no similarity, they are not even remotely close."

In light of the new information, the General Register Office amended the death certificate to include a footnote with the correct cause of death but left the original "guess" in the main cause of death box.

A 2003 study, carried out by experts at Queen Mary College in London, showed more than half of all death certificates contained inaccuracies.

Mrs Willis says this is unacceptable and misleading.

She said: "My family have been unable to accept being left with a death certificate which still notes causes of death which is far removed from the truth."

She has written to Prime Minister Gordon Brown and Conservative MP Henry Bellingham in a bid to push for a change in the law and for substantial training for doctors with regards to filling out death certificates. And it seems her efforts are paying off.

Drawing upon the case of Mr Willis, Mr Bellingham was due to meet with Government officials on Tuesday to debate a new bill which would see a medical examiner involved in the process of filling in a death certificate. Currently, the responsibility lies with a doctor or consultant who receives half a day's voluntary training on the issue.

"I feel very strongly that not only should a new death certificate be issued without the inaccuracies on them and only the true cause of death recorded, but these certificates with the "to the best of doctors knowledge" should not be issued in the first place," Mrs Willis said. "The far reaching implications of these types of death certificates are enormous, giving families false medical histories to pass on down the generations. We could all be living with a time bomb waiting to go off, without knowing the truth putting our family lives at risk.

"Many families would rather wait, I am sure, to have an accurate cause of death recorded and wait a few additional days for the funeral."