During his visit to Lewis, Corbyn was asked by reporters if he would welcome Donald Trump, whose mother was born on the island.

The Labour leader said he would rather see Jim Morrison, whose relatives also hailed from there.

Alan, who was married to Jim’s younger sister Anne, said: “I laughed my ass off when I read that.

“Of course, I can imagine why Jeremy Corbyn said that.

“Trump is so unpopular now in America. Even his supporters are souring on him.

“A poll last week showed that just a third of voters believe the president is drawing the country together. People now look fondly on George W Bush.”

Jim Morrison’s family tree shows that direct ancestor Alexander Morrison was born in Lewis in 1750.

But according to Alan, Jim’s family – his father George was an admiral in the US Navy – spoke very little about their roots.

Alan, 73, said: “The admiral said the family was originally from the Outer Hebrides. He also said he was a direct descendant of Robert the Bruce in an almost jokingly way – but there are some Bruces in the family tree. However, I don’t think they did research.

“Jim knew from his parents he was Scottish but never lived to see the research I did. I think he would have found it interesting. He might even have written about it or incorporated it into his poetry.”

Alan discovered that Alexander Morrison emigrated to America in April 1781 and settled in Hagerstown, Maryland.

Alexander’s wife is not named on records but he had three sons.

John was born in Lewis in 1780 and second son William was born just six weeks after the family arrived in America.

William married Sarah Slemmons, from Ireland, and their youngest son Stephen was born in 1826 in Butler County, Pennsylvania.

Stephen, who was Jim’s great-great- grandfather, fought on the Union side in the American Civil War, and took part of the Battle of Gettysburg, in which more than 50,000 were killed or injured.

A photograph of Stephen Morrison shows a remarkable resemblance to the Doors singer, who died in Paris in 1971, aged just 27.

Alan, author of the book I Remember Jim Morrison, said: “When I morphed the two photos together, I thought, ‘Wow!’ They both have the same amazing eyes, lips and nose. It blew my mind. I asked myself, ‘How could Jim look so much like his great-great-grandfather?’

“The answer must be strong genes in the Morrison family.”

Alan, originally from Liverpool, met Jim’s younger sister Anne in London where she was studying.

They married in 1966 and moved to the US two years later, where Alan met his brother-in-law for the first time.

Alan said: “He stepped off a flight on a hot Californian day wearing a World War II leather bomber jacket.

“He looked both ridiculous and cool at the same time. He was wearing aviator sunglasses with a green tint, Levi’s and cowboy boots, and carrying an antique briefcase stuffed with his writings.”

Alan said he got on well with his brother-in-law but added: “I hold the distinction for being the first guy to knock his ass out.

“He had a reputation for causing trouble wherever he went. Everyone wanted to beat him up but he met a kid from Liverpool who wouldn’t take his s***. I knocked the crap out of him and gave him two beautiful black eyes. He looked liked a panda for the next three weeks.”

Alan could see that the singer was spiralling out of control.

He said: “Jim became the greatest guy in the world after a few drinks. After a few more, he became the roughest guy in the world and you had to knock his ass out.

“He had a serious drinking problem from a young age, and when I met him he was well on his way to being 6ft under.