Paul Chamberlain is articulate, impossibly cheerful in the face of adversity and heroic – a Briton of the sort you might have thought would be seen no more after Lawrence Oates strode into the Antarctic snowstorm in an effort to save Scott and his other companions.

The comparison is not inappropriate. Chamberlain, who has travelled the world, cycled around Cuba and loves sport, was diagnosed three years ago with the progressive and terminal muscle-wasting motor neurone disease and now uses a wheelchair. He too is looking death in the face. And he wants to meet it with dignity.

Motor neurone disease does not offer an easy passing. "It is a lingering and very unpleasant death," he says matter of factly. "Your muscles waste away, you can't do anything, you end up possibly being fed through a tube, you have a machine to help you breathe, and you can be mentally competent – your brain is fully active – but you can't do anything. And that's not a very nice way to die."

Chamberlain, 66, a former chartered accountant who lives in the Surrey suburb of Walton-on-Thames, does not want to linger that long. Since the law forbids anybody to help him kill himself, he has made the necessary preparations to take his own life. He has obtained the drugs he needs from overseas, he says. He has to hope they are what he thinks they are.

He does not yet know when he will use them. MND progressively weakens the muscles until the sufferer cannot do anything by himself. Breathing has become difficult and Chamberlain is on a respirator 10-12 hours a day.

In the autumn, a parliamentary bill from Lord Falconer that would allow doctors in this country to help people with a terminal illness to die is expected to be debated, but even if it passes in the Lords, it still faces the greater hurdle of the Commons and any change in the law will be too late for Chamberlain.

He is giving his first interview about his intention to kill himself to the Guardian partly because he thinks the present law is wrong and partly to protect Sally, his wife. He wants to make his intentions quite clear so that no suspicion can fall on her after his death.

"Knowing what happens to people with motor neurone disease, an early thought was I'd rather not go the full term. I don't see any point in having the last six months laying there, being unable to communicate, being unable to move, hoisted on to the toilet or wearing diapers. I don't really see a lot of fun in that. I understand some people live their whole lives like that and I admire them for doing so, but I don't think it's for me," he says.

"I have obtained something which I believe will do the job. I'm not ready to take it yet, but I will be at some stage – I'm not quite sure when. In my mind, I thought if I cannot get myself to the toilet, that will be enough, because it's only going to go downhill from there. It's only going to get worse … There is no cure. I know what's going to happen, the neurologist knows what's going to happen and I'd just rather avoid for myself and for the family seeing myself in a fairly pitiful state."

Chamberlain has joined the organisation Dignity in Dying and supports the Falconer bill. He says that in the Netherlands, Belgium and the state of Oregon in the US, where there are assisted dying laws, there has been no evidence of abuse. "I know the argument that you persuade Auntie Doris to take her life because then you inherit the money, but you've got to have two doctors to certify that you are mentally competent and you are making a rational, well-informed decision on your own, without persuasion," he says.

"I understand there is some opposition to Lord Falconer's bill and some lords may strongly oppose it, but I have a question for them and for anyone else who opposes the bill. I'd like to know if they could look me in the eye and tell me that they are determined to ensure that I have a lingering and very unpleasant death. Because no matter how good the palliative care, it is a lingering and unpleasant death."

As it is, with no doctor to help he will have to take his chances with the drugs he has, which may not work or may have unexpected effects. "I know that would be horrible, but no more horrible than what I'm facing anyway," he says.

The first Chamberlain knew that anything was wrong was when he had a couple of falls on the tennis court. He blamed it on an arthritic ankle. He went to the club physiotherapist for exercises to strengthen it and was told "you have foot-drop" and advised to see his GP immediately. The GP sent him to a neurosurgeon, who sent him to a neurologist.

His official diagnosis came on 5 August 2010, although he thought he knew a month earlier. But he and Sally told nobody for some weeks in order to avoid spoiling their son's wedding.

"It was quite difficult at first because we've got quite an active social life and we were going to parties and events and I had this sort of dreadful secret. We were supposed to be having a nice time – and we did have nice times – but it was very difficult at first. And there were some emotional ups and downs in the early days and on one very brief occasion there were some tears. But it wasn't really about me – it was not being there to help my sons in the future. That was the sad thought."

They have two sons and a grandson aged two and another grandchild due in November, whom he thinks he may never see. His family members are all very supportive and respect his decision, he says.

Chamberlain himself remains extraordinarily upbeat. "I've never dwelled on what I can't do. I think about what I can do," he says. "And we still go out. I'm having a sandwich lunch and playing bridge this afternoon at the tennis club, we're meeting friends for supper at the pub tonight, we're going to a bridge supper on Thursday night, we're having our 42nd anniversary lunch with some friends on Sunday. We do a lot. It takes a lot of effort to get ready but we still go out and about. And there's no point in being miserable. I think you've just got to accept what's happening really."

He has had, he says, a good life. "We've travelled a lot. We've had a long and happy marriage. We've got two lovely boys and a grandson and one on the way. I had a reasonable career. I've had lots of friends," he says.

And now, all he asks is to be sure that he can have a good death.

Samaritans are available around the clock, every day of the year, for anyone struggling to cope. To find your nearest branch, call 08457 909090, visit samaritans.org or email jo@samaritans.org