Introducing assisted dying is a matter of "when not if", according to the MSP who is spearheading a new bill to allow terminally ill people to choose when to die.

Liam McArthur's proposed members bill would give terminally ill patients, with mental capacity, the right to access assistance to end their lives rather than being forced to travel overseas to do so.

A new bill to introduce assisted dying in Scotland is due to be published by the Scottish Parliament later this year, in what will be a third attempt to legalise the option.

The Liberal Democrat said: "My certainty comes from hearing so many cases of individuals who are being let down by the current ban on assisted dying.

"It comes from seeing how other countries and states around the world have been able to introduce this without the sky falling in, without the slippery slopes, without a wholesale exodus from the medical profession we're told would be the case here in Scotland.

"Over time I have become more certain of it.

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"It is one of these situations where once we change the law - and I believe we will - we will wonder why on earth it took us so bloody long to get to this point."

Polls consistently find that Scottish voters are in support of proposals to give a choice to terminally ill people to end their lives with a recent poll, in September, finding that 77% were in support.

Some 12% of those who were question opposed the idea of the proposed Assisted Dying for Terminally Ill Adults (Scotland) Bill.

The legislation would allow people aged 16 and over who have been told they have less than six months to live, and who are of competent mind, to end their lives as long as they are able to administer and ingest the medication themselves.

There is strong opposition to the proposals, in particular from faith groups, with concerns that legalisation of assisted dying may lead to the accessibility criteria being widened to what they view as an intolerable degree, and issues of coercion.

A joint statement from the Church of Scotland, the Catholic Church and the Scottish Association of Mosques previously stated that "assisted dying inevitably undermines the dignity of the human person, and to allow it would mean that our society as a whole loses its common humanity."

Canada, where medical assisted dying (MAID) is legalised, has been grappling with issues of eligibility recently as the country prepares to expand its laws to include those with mental illness.

Canadians, including doctors, are concerned the assisted death programme has accelerated too rapidly.

Mr McArthur said: "We have a different constitutional relationship here in the UK than in Canada and I think that while no two laws are the same anywhere in the world they fall into those two broad categories, the expansive one of Canada and the Netherlands and a terminal illness bill, which is the one Oregon adopted and has been picked up by 10 and 11 states in the US.

"Oregon's law has not changed in terms of eligibility criteria in 25 years.

"So this argument of the inevitability of the slippery slope is debunked by the fact you've got these laws that have been in place longer than any others that really have not changed in terms of the breadth and scope and that's the model I'm proposing to introduce in Scotland."

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Mr McArthur, MSP for Orkney, emphasises that the bill must be "properly safeguarded and compassionate".

There will be no whip applied when the legislation comes to a vote - MSPs will be given a free vote and do not need to vote along party lines.

Mr McArthur is hopeful that the debate will be respectful, as he says discussion has been throughout the lengthy and multiple efforts to move to this stage.

It is a third attempt to introduce assisted dying legislation; a previous bill sponsored by Margo MacDonald MSP, who lived with Parkinson's disease, and taken up by Patrick Harvie MSP on Margo's death, was was defeated in a First Stage debate in 2015.

Prior to that Jeremy Purvis MSP had proposed a bill in 2005 but it failed to gain the 18 supporters needed to progress it.

Mr McArthur said: "I well remember sitting in on the debate by my former colleague Jeremy Purvis when he was trying to introduce a bill based on the Oregon model which resulted in Margot listening in on that debate and deciding to bring in a bill of her own.

"I think it is exactly the kind of debate that this parliament was set up to discuss.

"At that time it seemed inconceivable that a bill of this kind would be introduced any time soon but when Patrick took up the bill for a second time I remember arguing passionately for a change in the law.

"Sitting behind me was my colleague Alison McInnes who put forward an equal passionate argument against it.

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"And at the time it felt right and appropriate that the people were arguing from a position of deep conviction and passion but doing so in a way that was respectful - and I hope we can do that again."

While he knows there are people in the parliament who will not be persuaded that change is needed, the MSP says he hopes they will engage with the debate nonetheless.

He added: "I am interested in how not whether we change the law.

"I'm not willing to have a debate about whether we need to change the law because I fully believe we do - and we will wondering in time why it took us so long - but I don't want those who have concerns to feel as if just because they find themselves on that side of the argument that they don't have an influence and can't help to shape it."

Mr McArthur added that Scotland "must do better" to support people in end of life care who want to utilise assisted dying.

Currently people might travel to the Dignitas clinic in Switzerland but, the politician said, the position is "cowardly".

He said: "Essentially we are outsourcing the issue by saying if you have got the financial resources and the physical wherewithall to get to Switzerland then we are quite happy for you to go. It's cowardly, we need to take ownership of this.

"If you speak to people at Dignitas there is a sense of frustration that there are countries like Scotland that our outsourcing the support at the end of their life that they need and outsourcing it to those who have the financial and physical ability to go.

"They are opting for assisted death earlier than they would do in Scotland.

"An outright ban is utterly unsustainable and doing nothing has consequences and that's what opponents of this bill have to recognise."