A hospital in Melbourne has offered an apology to a patient for declaring him dead.

Peter Thornton was admitted to the Alfred Hospital in early March for a severe bout of gastro.

He checked himself out after he says he was left lying on a trolley for 12 hours.

On visiting his GP yesterday, he was shown a letter from the Alfred stating he died on March 9.

"I can't believe that a doctor has declared me dead," he said.

Mr Thornton's GP, Dr Beng Eu, says he was shocked to receive the letter.

"This has never happened before, I've never come across this before in all the years I've practised," he said.

The Alfred Hospital has told the ABC it will provide Mr Thornton with a full explanation as soon as an investigation is complete.