Howard Marks, the notorious former dope smuggler known as Mr Nice, has told the Observer he has been diagnosed with inoperable bowel cancer.

“I’ve come to terms with it in my own way – which for me was about learning how to cry,” Marks, 69, said this weekend. “It’s impossible to regret any part of my life when I feel happy and I am happy now, so I don’t have any regrets and have not had any for a very long time.”

Marks, a long-time campaigner for the legalisation of cannabis, has already received messages of support from his wide-ranging group of friends. Artist Tracey Emin, the journalist Peter Hitchens, Sir Richard Branson, broadcaster Zoe Ball and DJ husband Norman Cook have all sent best wishes this weekend. Hitchens has praised Marks as a “principled defender of free speech” despite taking an opposing side on the legalisation debate. Cook and Ball have described him has “a ray of sunshine in our lives”.

Marks, who spent seven years in a high-security prison in America, has been raised by fans of his bestselling biography, Mr Nice, to the position of entertaining veteran of the drug-fuelled hippy scene of the 1970s. His confessional memoir, published in 1996, made Marks a household name and went on to sell more than a million copies.

He then embarked on a performing career, touring Britain to talk to audiences about a life that had taken him all over the world and involved him with the mafia, the Palestine Liberation Organisation, the IRA and security services on both sides of the Atlantic. In 2006 he became the subject of a film, also called Mr Nice, in which he was played by his friend and fellow Welshman, Rhys Ifans.

This weekend Ifans confirmed he would be involved in a celebratory concert in north London. The starry event at the Kentish Town Forum on 27 February will see Ifans joined on stage by Marks, members of the Welsh band Super Furry Animals and DJ Greg Wilson, alongside many other musicians. Funds raised will help Marks and his dependants through his cancer treatment, set up a charitable foundation and complete a documentary about Marks, which has been shot over the last two years. Documentary-maker Sam Rowland has been given access to unseen home-movie footage chronicling Marks’s unconventional life.

Another of Marks’s friends, the actor Keith Allen, reacted with sorrow and warmth when he heard the news. “Howard should get an OBE for keeping Britain stoned in the 70s and 80s and a knighthood for what he’s done since he came out of prison,” said Allen. “He is one of the cleverest, nicest and most charming old rogues I have ever had the pleasure of spending time with. He is a Welsh national institution and a national hero and will long be cherished as such. On second thoughts, he should probably turn down the knighthood.”

Rhys Ifans as Marks in the film of Mr Nice. Photograph: PR

Cancer was diagnosed last autumn and Marks was told nothing could be done to stop the disease, which has spread to his liver and lungs. Medical sources told the Observer that Marks had completed eight cycles of chemotherapy and responded well. Marks, who has four children and an ex-wife, Judy, plans to battle on with his call for cannabis to be legalised. “Of course the legalising of marijuana for medical purposes is to be welcomed,” he said, “but personally I never wanted to have to wait until I had cancer before I could legally smoke. I want it to be legalised for consuming recreationally – and I’m pleased to see they have now done this in four US states. After my experiences at the hands of the US legal system, America is the last place in the world that I thought would be leading the charge.”

Marks, whose many former aliases include Narco Polo and who was famously described as sounding like Richard Burton and looking like a Rolling Stone, is receiving treatment in his hometown of Leeds and being cared for by his long-term partner, Caroline Brown.

“I’ve never cried before. In prison I cried deep in myself, but I had to be the tough guy, I couldn’t let any vulnerability show,” Marks said. “But then I think, how long am I going to be living for anyway? I don’t want to be living until I’m 350! The strange thing is I haven’t actually felt a moment’s depression at all. I feel people have come back from a lot worse than this, and it’s about trying to resume a normal routine as soon as possible … and just carry on living.”

Born near Bridgend to a naval officer and a teacher, he studied physics at Balliol College, Oxford, and was a contemporary of Lord Patten, the former chairman of the BBC Trust and governor of Hong Kong, of whom Marks said: “I used to let him sneak his girlfriend, Lavender, through my window.”

Marks remains unrepentant about his life in drugs, but said he is sorry for the grief it brought his wife and children. “Smuggling cannabis was a wonderful way of living – perpetual culture shock, absurd amounts of money, and the comforting knowledge of getting so many people stoned,” he said.

“Prison took the wind out of the sails for a while, and was a terrible blow to my young family. However, I felt immensely lucky and privileged to have written a bestseller and then embarked on a career of performing in public, which allowed me to hang out with musicians, DJs and other lovers of dope. I have had an incredible life.”

LIFE AND TIMES

Dennis Howard Marks was born in August 1945 in Kenfig Hill, south Wales. Father Dennis was in the merchant navy; mother Edna was a teacher. After grammar school, he read physics at Oxford, where he became involved with drugs.

As a drug smuggler in the 1980s, Marks had 43 aliases, 89 phone lines and 25 companies trading worldwide as fronts for money laundering.

Sentenced to 25 years for drug smuggling, he was released in 1995, after seven years, for good behaviour. His 1996 autobiography, Mr Nice, sold more than a million copies and was made into a film starring Rhys Ifans, below. Marks became a festival regular and campaigned for the legalisation of cannabis.

In 1996 Super Furry Animals released a song called Hanging With Howard Marks.

Divorced, he has four children. His partner is Caroline Brown.