A British Airways co-pilot who believed he had been poisoned by contaminated cockpit air had been suffering from a nervous system problem which could have caused his "excruciating" symptoms, an inquest has heard.

Neuropathologist Dr Daniel du Plessis told the hearing in Salisbury that Richard Westgate died as a result of an overdose of the insomnia drug pentobarbital but added there was no evidence it was intentional.

He added that narrowing of the arteries and lymphocytic myocarditis - an inflammation of the heart muscle - might have "quickened" Mr Westgate's death.

The 43-year-old, from Marlborough,  died at the Bastion Hotel in Bussum, Netherlands, in December 2012 while undergoing treatment after he suffered numerous symptoms which he put down to "aerotoxic syndrome".

Dr du Plessis, of the Greater Manchester Neuroscience Centre, said he found evidence that Mr Westgate had been suffering from neuritis - inflammation of the nerve roots - which could explain many of these symptoms including pain, tingling sensations, numbness and balance problems.

He said: "It establishes beyond any doubt Mr Westgate was suffering from damage to his nervous system and that can explain a lot of the symptoms he had."

Dr du Plessis said that organophosphate poisoning - the supposed cause of aerotoxic syndrome - could cause nerve problems but not the nerve inflammation suffered by Mr Westgate.

However, he added that he could not rule out this being caused by an auto-immune response to the poisoning.

He said: "I cannot dismiss it, it needs further investigation, the current scientific evidence is limited."

Dr du Plessis added that a more likely explanation for Mr Westgate's condition was an autoimmune condition such as Sjogren's disease, which causes the immune system to start attacking healthy cells and tissues.

At the start of the week-long inquest, coroner Dr Simon Fox ruled that aerotoxic syndrome would not be treated as a factor in the death of Mr Westgate.

Lobby group Global Cabin Air Quality Executive is campaigning for equipment to be installed in aircraft to monitor air quality, which is supported by unions representing some airline workers.

In 2015, the Unite union launched a helpline and revealed 17 personal injury claims had been made involving former and current cabin crew staff working for UK airlines.

But the industry insists that incidents of smoke or fumes on planes are rare and there is no evidence of long-term health effects.

On Thursday his twin brother, Guy Westgate, told the Salisbury inquest, that his brother had been in "excruciating pain" as he suffered from various symptoms.

These included digestive problems, fatigue, general pain, headaches, loss of cognitive ability, clumsiness and inability to sense temperature.

The 47-year-old, who is also a BA pilot, said his brother felt let down and angered by doctors and specialists in Britain, including those from his employer BA, and went to Hilversum in Holland for treatment.

He said: "The specialists in Holland were the first group of specialists who gave him a light at the end of the tunnel that they believed in him. He went there to be cured."

Fighting back tears, he added: "He would describe pain in his brain, he felt his head had been tin-opened and his brain had been sandpapered. It was the only way he could describe the severity of his pain."

Mr Westgate said: "We do not believe it was suicide. We were particularly surprised as we were planning to speak to him the following day."

When asked by David Platt QC, representing BA, if a road traffic accident in August 2011 had led to his brother's symptoms, Mr Westgate said his condition pre-dated the crash.

Mr Westgate said his brother, who had suffered whiplash, had used the crash as an acceptable way of getting signed off from work to claim on insurance, which he had otherwise been unable to do without a formal diagnosis of aerotoxic syndrome.

He said: "Richard needed money to fund his treatment in Hilversum.

"Everyone realised if you asked a BA doctor to make an inconvenient conclusion of organophosphate poisoning, of aerotoxic syndrome, they would never agree to that.

"Everyone was trying to get Richard a convenient diagnosis. We were all trying to attach what symptoms we could (to the accident) to make it easy for Richard to start his loss of licence payout."

Mr Westgate added that he had been unaware that his brother used the pseudonym of Richard West to seek treatment for depression at the Priory Clinic in Bristol.

The hearing continues