Friday’s vote is just the latest attempt to introduce assisted dying – it was first debated in Parliament in 1936.
The current bill – called the Terminally Ill Adults (End of Life) Bill – has been introduced by the Labour MP Kim Leadbeater.
She came top of a ballot of MPs and so her bill – known as a Private Members’ Bill – is the first to be considered and has probably the best chance of becoming law.
Even though the government has remained neutral on the issue, and MPs can vote according to their own beliefs, ministers have already come out in favour or against the bill.
For Sir Nicholas Mostyn, a retired High Court judge, the compassionate thing to do would be to give him the choice to end his life before his body deteriorates to the point he can no longer physically do everyday tasks.
Like Mark, he has also been diagnosed with Parkinson’s but he is not yet in the advanced stages of the disease.
“The likelihood, if you’ve got Parkinson’s disease, your ending is going to be prolonged and very unpleasant,” he tells BBC News. He supports the bill – even though it would not give him the right to end his life.
Symptoms of Parkinson’s include parts of the body shaking uncontrollably and slow movement. In the most advanced stages, the disease people can find themselves unable to move and unable to speak.
Sir Nicholas, and some sufferers of other debilitating conditions not considered terminal illnesses, would like the bill to be amended to cover them.
For some critics, this is an important reason to vote against it.
They fear, whether now or in the future, this bill could be widened to include sufferers of non-terminal conditions – this, they say, would be a danger to disabled people.
The example most regularly cited is Canada, which opponents say is an example of a so-called “slippery slope”.
Legislation introduced there in 2016 was initially just for the terminally ill, but was extended in 2021 to those experiencing “unbearable suffering” from an irreversible illness or disability. There have been delays to further extensions, but it is still due to become available to those with a mental illness in three years.
Sir Nicholas says: “I just don’t understand the moral argument, which is that because I wish to exercise sovereignty over my own body, that I am in some way facilitating a ‘slippery slope’ for abusive treatment of people who don’t actually want to [end their lives].”
Friday’s vote – if it passes – would just be the beginning of a long parliamentary process; weeks of scrutiny by a committee of MPs will follow, as they go through the legislation line-by-line.
The bill will then return to the House of Commons and then the House of Lords where it could be amended in further votes.
Even if MPs vote in favour of the bill – there is still a long way to go before these proposed changes become law.
But if they do, it will mark another significant reform of the law that has seen our society change so much over the past 50 years.