The 21-year-old, who is studying to be a doctor in Glasgow, said he was "very much" in favour of an opt-out system of organ donation, where the default position is that everyone is a donor.
James, the runner-up of last year's BBC show, said having to make an active decision to join the register was deterring many people, who are otherwise support-ive of organ donation.
Meanwhile, he said hundreds of people were living, a "poor quality of life" waiting for a transplant.
James endorsed Opt Out as he helped promote the NHS 'Wee Chat' campaign to get people talking about organ donation.
He admitted he was in favour of the Evening Times campaign to persuade the Scottish Government to introduce an opt-out system, which is due to be implemented in Wales in 2015, and has attracted around 18,000 signatures of support.
He said: "I'm very pro opt-out. For those of us in the medicine world, we are constantly faced with statistics that show there are just not enough people donating.
"I am sure most people would agree with it.
"At the moment you have to make an active choice to donate which I think puts a lot of people off. Most people just don't get round to it."
However, James, who lives in the city's West End, said while an opt-in system was in operation we had to do everything possible to drive up numbers including speaking to family members about your wishes.
He was joined by Hannah Mac-Kereth, who is celebrating the 20th anniversary of receiving a life-saving liver transplant, in city centre tearoom Cup, to promote the Scottish Government's campaign to increase numbers.
Wearing one of the trademark Fair Isle sweaters he was known for on the hit BBC show, James said he is hoping to combine his career in medicine with cookery writing.
A new book about baking bread is due to hit the shelves later this year. James, the son of BBC Radio Scotland host Tom Morton, said: "I really enjoy writing and it's something I can do fairly quickly.
THAT'S the good think about baking it stays with you. I'm trying to keep it up as much as possible."
But he admitted that the last cake he baked was a "disaster". He said: "It was very experimental."
James was known for his innovative, slightly slap-dash approach to baking which host Paul Hollywood described as a "stroke of genius".
He revealed it was his girlfriend Fenella Barlow-Pay's 23rd birthday but a friend of his had bravely offered to put herself up for scrutiny by baking a carrot cake for her.
She is also studying medicine.
James is now in his third year at university and is just about to start a placement on the wards of the Victoria Infirmary.
He said: "I'm hoping the nurses will teach me stuff if I bribe them with cakes.
"I'd be happy to be a GP and I'd be happy to do surgery.
"I'll need to start thinking about tailoring my CV."
Meanwhile, he is happy to use his celebrity credentials to support worthwhile causes, including driving up organ donation rates.
He said: "Organ donation is one of the most altruistic things anyone can do.
"In most walks of life, there would be someone else to fill your role.
"If I hadn't been on the Bake Off, someone else would have taken my place. But organ donation is one important exception.
"Your decision can literally save lives and that's why it's important to talk about it, with or without cake."
WHEN Hannah MacKereth was five-months-old her parents were given a stark choice.
Diagnosed with a life-threatening condition which was destroying her liver, doctors told Jackie and Paul they could put her on the transplant list with no guarantees an organ would come in time or "let her drift away".
Like most parents they opted for the chance to save her life.
Thankfully, the call came within three months. Hannah was one of the first to receive a split liver transplant, where an donor organ is divided to help multiple patients.
Surgeons were able to save the lives of around seven people with the organ which had come from a teenage car crash victim.
Next month on February 5, Hannah, from Bishopbriggs, will celebrate the 20th anniversary of her transplant.
She is planning a celebratory party for family and friends. And everyone who is invited will be "gently pushed" to sign the organ donor register.
She said: "If it wasn't for an organ donor I wouldn't be sitting here today.
"Organ donation gave me another chance at life when there was no hope.
"There are no words to describe the gratitude I feel for the family."
Hannah was born on June 15, 1992, suffering from jaundice, something that is common in newborns.
However, at five-months-old a specialist demanded tests after noticing how yellow her skin was.
She was diagnosed with Biliary Atresia, a life-threatening condition in infants in which the bile ducts inside or outside the liver do not have normal openings.
Bile is a fluid made by the liver that carries toxins and waste products out of the body. An operation to repair her liver failed.
Hannah, who has an 18-year-old sister Olivia, said: "My mum was told they could put me on the donor list or let me drift off into sleep.
"There was no choice obviously. I was lucky I didn't have to wait long."
After the operation Hannah made a complete recovery and now only needs a small amount of anti-rejection drugs. She is studying Physiology at Glasgow University and hopes to go into medicine.
Scotland is continuing to lead the way in donor numbers with more than 41% on the register.
However there are more than 600 people waiting for a transplant.
Hannah is very supportive of an opt-out system and any other measures that could help drive up numbers.
She said: "I just can't understand why anyone would object."