Dr Sergio Canavero, an Italian neurosurgeon, says he is getting ready to carry out the first ever human head transplant. He has already found a volunteer for the controversial operation - terminally ill Valery Spiridonov from Russia, who says he wants to help in any way he can to make a "huge scientific breakthrough".

However, questions remain over the feasibility and the ethics of the operation. Canavero says he is preparing to carry out the operation within two years and according to his estimates, the procedure would see Spiridonov's head being attached to the body of a donor through spinal cord fusion (SCF).

It is an operation that will require a team of more than 100 medical workers and could take 36 hours to complete.

According to Canavero's calculations the operation would cost around $15m and would take place either in China or the US.

Canavero has also been quick to dismiss critics who do not believe the operation will work and has suggested anyone opposed to the idea should travel to Valery's hometown in the Russian city of Vladimir, east of Moscow, to see for themselves.

"No problem. Come to Vladimir, Russia. I will tie you up to Valery's wheelchair, you will poo and pee the way he does, you will sleep the way he does, for 24 hours, OK? After 24 hours, I will ask you 'do you still believe that this is going to be a crazy project?' and I can bet you 100 to one that it is possible he will change his mind," he told Reuters in his studio in the northern Italian city of Turin.

Canavero says the key to the procedure is a sharp severance of the spinal cords. The head needs to be removed with a sharp blade, causing minimal damage to the spinal cords. Speed, too, is of essence, he says.

"Actually, as everybody knows by now, the head will be cooled and there won't be a single drop of blood inside so he will be clinically dead, as dead as it gets as I said in my TEDx talk. And actually this momentary absence of circulation is, it's actually momentary because the two gurneys will be like this in the same room, so the head will gravitate for just a few seconds until the surgeon will start reconnecting the head to the new body," he said.

Canavero, who is still seeking funding for the project, is due to address the American Academy of Neurological and Orthopaedic Surgeons' 39th Annual Conference in Annapolis, Maryland, in June.

Dr Christopher J Winfree is an assistant professor of Neurological Surgery at the College of Physicians and Surgeons in New York City. He says most physicians agree on the ethical, technical and physiological limitations that a head transplant procedure poses.

Winfree said the main concern that had arisen in his talks with his colleagues was reattaching a severed spinal cord.

"The real issue is when we get down to the physiology of the situation. One of the problems that is brought up is the ability to heal the severed spinal cord, which is what would happen if you cut a head off and put it on a different body. You would try and fuse that spinal cord together. Our current technology doesn't allow that healing to occur," he said.

Spiridonov now speaks regularly with Canavero via videophone and admits he is scared about the procedure but says time is running out for him.

The computer scientist suffers from Werdnig-Hoffman disease, a rare form of spinal muscular atrophy, which causes severe muscle weakness. Life expectancy for his type is low, with few sufferers reaching adolescence or young adulthood. At 30, he says his condition is rapidly deteriorating.

"Valery is an incredible human being as I said many times over. Valery will go down in history as the 24th century's Yuri Gagarin because he will open up a new frontier and new scenarios which right now we couldn't even begin to fathom," said Canavero, referring to Russia's first man on the moon.

While not having met in person, Canavero and Spiridonov share a common urgency for the procedure to take place.

"Ok, I'm very happy to participate in this project and I think it will be a huge breakthrough, a science breakthrough. I will be the first in this experiment. I totally volunteered in it and I want to help in any way I can," said Spiridonov, speaking to Canavero via a videocall.

Canavero says his ideal date for the operation would be Christmas Day in 2017. It would be a nice present, he said.