A British backpacker who believed he had been poisoned from a spider bite died in police custody in Thailand after officers thought he was "insane", an inquest jury has been told.

Three weeks before his death, Daniel Salmon, 32, told his mother on the phone he believed he had been bitten on the hand by a spider, which had caused his arm to swell up as far as the elbow.

After deliberating for around 20 minutes, jurors at Worthing County Court in West Sussex recorded a verdict of death by natural causes.

Maureen Salmon, 64, told the inquest she had been alarmed as he had appeared confused and slurred his speech and during further phone conversations she had worried he was behaving strangely.

But she said despite receiving several conflicting reports from the Thai authorities since her son's death on April 14 2007, none acknowledged the bite.

A Bangkok post-mortem examination found Mr Salmon, a technician from Lancing, died of failure of respiratory and blood circulation systems, but Dr Mark Appleton told the inquest this was not a conclusive cause of death.

Forensic pathologist Dr Appleton, who did not receive the report until August last year, said he suspected much of it had been lost when it was translated from Thai into English and some parts were missing while others were illegible.

"Unfortunately it's not really possible to come to a conclusion as to what the cause of death might have been," he added.

Mrs Salmon said her son, known as Danny, had spent nine months backpacking around Australia the previous year. He was nearing the end of a two-month trip around South East Asia when he died.

He believed he was bitten in Laos on March 22. He was given anti-toxic treatment but could not be prescribed an anti-venom as he could not identify the spider.