An underground graffiti artist who had a year-long feud with stencil artist Banksy died three years after he was found lying unconscious in a pool of blood outside his King’s Cross flat, an inquest heard.

John Robertson, who was known to the art world as King Robbo, died of pneumonia at a neurological centre in Hertfordshire on July 31 having been in a vegetative state after sustaining a brain injury outside his flat in Calshot Street in April 2011.

On Wednesday, coroner Mary Hassell told an inquest into Mr Robertson’s death at St Pancras Coroners’ Court that there was “no suggestion of foul play” but concluded that there was “scant evidence” to explain how Mr Robertson sustained his injury.

The 44-year-old was found lying at the bottom of a flight of 10 steps with a fractured skull in the early hours of April 2, 2011, having earlier been spotted walking up the steps while looking at his mobile phone.

Mr Robertson made global headlines as King Robbo in 2009 when world-famous stencil artist Banksy defaced one of his most historic works on a wall beside Regent’s Canal, beneath the British Transport Police headquarters in Camden Road, Camden Town.

This led to a series of tit-for-tat overpaintings from the two artists on the wall, originally sprayed by King Robbo in 1984, over the following year.

Mr Robertson’s true identity was only uncovered after his death having never revealed his face or real name during an art career as King Robbo spanning three decades.

Following his death, graffiti artist Doze, 48, a close friend, described him as “the most prolific and respected British graffiti artist ever”.

The feud with Banksy, which brought King Robbo out of retirement, became the subject of a Channel 4 documentary filmed shortly before his head injury.

Banksy publicly objected to the programme, Graffiti Wars, saying it inferred he might have had something to do with his rival’s accident, which the broadcaster denied.

On Wednesday, the court heard Mr Robertson had been drinking alcohol on the evening of his death and was later found lying outside his flat by paramedics who were called by a passer-by in a taxi.

He was rushed to the Royal London Hospital, in Whitechapel, where he underwent surgery to treat a bleed on the brain.

The father-of-three was eventually moved to the Whittington Hospital, in Archway, and then the Royal Hospital for Neuro-disability, in Putney, where he was diagnosed as being in a vegetative state in March 2012.

In March this year, he was moved to Gardens and Jacobs neurological centres, in Hertfordshire, where he passed away on July 31 having contracted pneumonia.

Recording an open verdict, Ms Hassell said: “I have no hesitation in saying Mr Robertson died as a result of the long-term consequences of the traumatic brain injury he suffered in 2011.

“However I have scant evidence as to how that brain injury was sustained. He was found at the bottom of a flight of stairs and it may be that he fell down those stairs. I have no evidence of that.”