Proposed legislation to allow assisted dying in Scotland is outside Holyrood’s powers, the Scottish government has claimed.

Health Secretary Neil Gray said issues relating to lethal drugs were reserved to Westminster.

He said it was the government’s view that “the bill in its current form is outside the legislative competence of the Scottish Parliament”.

Liberal Democrat MSP Liam McArthur, who has tabled the bill, said there was “no reason” that existing mechanisms allowing for powers to be transferred to the Scottish Parliament could not be used in this case.

The presiding officer – who decides which legislation is within the competence of the Scottish Parliament – has previously said she is confident the bill is within the powers of Holyrood.

The Assisted Dying for Terminally Ill Adults (Scotland) Bill, if passed by MSPs, would allow terminally ill adults to request assistance to end their life.

To be eligible, applicants would have to be resident in Scotland for at least 12 months, registered with a GP in Scotland, and have the mental capacity to make the request.

In a letter to the health committee, Gray raised particular concern about a section that would allow ministers to specify an “approved substance” to be provided to terminally ill people to end their own life.

Gray said that section appeared “to relate to the reserved matter of medicines, medical supplies and poisons” in the Scotland Act.

The health secretary said several other areas of the bill could relate to reserved regulations of the health professions due to a “fundamental shift” in the role of medical practitioners from “protecting/enhancing patients’ lives to assisting in termination of life”.

He highlighted a provision of the bill that would allow ministers to specify what qualifications and experience medics would need to be allowed to be involved in assisted dying, as well as a provision that would require medics to take into account a second opinion from a colleague on a termination request.

Gray also said a section of the bill that would grant a right to conscientious objection could also be outside Holyrood’s devolved powers.

The bill’s policy memorandum notes the possibility of using orders under the Scotland Act 1998, such as a section 30 Order, to implement the bill.

A Section 30 order is a piece of secondary legislation that can be used to increase or decrease – temporarily or permanently – the Scottish Parliament’s legislative authority.

Such an order was used to allow Hoylrood to stage the 2014 Scottish independence referendum.

The government said “further processes would have to be gone through in order to bring the bill within competence”.

Gray said that the process of a Section 30 order generally takes 12-18 months and requires the co-operation of the UK government.

McArthur said: “The Presiding Officer has certified that the Scottish Parliament can pass a bill in this area but I have always been clear that the Scottish and UK governments will need to agree to measures that will ensure that a truly comprehensive assisted dying process can operate in Scotland.

“There are established mechanisms for transferring powers to enable legislation to be fully enacted in Scotland and no reason why these cannot be applied in this case.”

He added: “Our current laws on assisted dying are failing too many terminally ill Scots, often leaving them facing an undignified and sometimes painful death despite the very best efforts of palliative care.”

The government has pledged that SNP MSPs will be allowed a free vote on the issue when it comes before parliament.

At stage one, MSPs will vote on general principles of the proposals, which must be agreed if it is to progress to stage two. The bill must then pass a further stage before it can become law.

Opponents to the legislation have raised concerns that some terminally ill people could feel under pressure to end their lives.

Two previous attempts to pass bills on assisted dying in 2010 and 2013 – both by the late Margo MacDonald, who had Parkinson’s disease – were rejected at stage one.