The Scottish Government has taken an "absolutist, fundamentalist" position by barring prisoners from voting in next year's independence referendum, a former Nationalist MP has claimed.

Margo MacDonald, an independent MSP who was previously an SNP MP, questioned why the issue is being debated at Holyrood because she cannot see how the Government could change its stance.

Deputy First Minister Nicola Sturgeon said ministers are "not persuaded" that convicted prisoners should be allowed to vote in the historic ballot on September 18 next year.

But she pledged they would "listen carefully" to the arguments.

She spoke out after being challenged on the issue as MSPs debated the general principles of the Scottish Independence Referendum (Franchise) Bill.

The legislation sets out who will be able to vote, with Ms Sturgeon describing the Bill as "the first important legislative step towards the referendum".

On prisoner voting, Liberal Democrat MSP Alison McInnes said that for such a crucial ballot, it "seems reasonable to take a measured and mature look at exactly who should be allowed to participate in that decision".

She asked the Deputy First Minister: "Is she open to exploring participation of some prisoners? And if not could the cabinet secretary explain how she thinks excluding people from this most high profile and significant choice is will help with reintegration and civic responsibility?"

But Ms MacDonald said Ms Sturgeon has "set out an absolutist, fundamentalist position".

The Lothians MSP said: "I cannot see how the Government can move from it, therefore I wonder why we bother debating it."

Ms Sturgeon had told Parliament: "It is the position of the Scottish Government that convicted prisoners should not be entitled to vote in the referendum. The franchise for the referendum is based on the franchise for local government and Scottish Parliament elections, neither of which currently extend to convicted prisoners.

"The reason for that, in my view, is that if somebody commits a crime which lands them in prison, they break their contract with society. Therefore they lose some of the rights the rest of us take for granted. That, as the law stands just now, includes the right to vote in elections.

"We are not persuaded of the arguments to change the status quo position but will listen very carefully to the arguments being made."

The Bill would give 16 and 17-year-olds the right to vote next year and is "an essential part of the framework within which the referendum will be held", she said.

"The Bill provides for a franchise which is based on the existing franchise for the Scottish Parliament and local government elections, with one change. That change is the proposal that the franchise is extended to 16 and 17-year-olds.

"Young people take on a number of other important rights and responsibilities at 16 and I believe it is only right they should be fully engaged in the democratic process."

Labour MSP Patrician Ferguson objected to calls for prisoners to have a vote.

"I think the threshold is probably going to be too high," she said.

"The kind of prisoner we're talking about is not the kind of prisoner that's committed a low-level offence. It's someone who's committed a fairly serious offence and I would, in principle, object to them having the vote in any situation," she said.

She agreed that younger people should have a say but called for a solution so children of service people abroad can vote.

"We would hope that the Scottish Government will work with the UK Government to ensure that the children of service personell are in that list," she said.

Conservative MSP Annabel Goldie said prisoners should lose the right to vote when they commit an offence.

"In my opinion, convicted prisoners serving custodial sentences have, by committing their crimes, sacrificed their entitlement to vote," she said.

"The best way to preserve that entitlement is in fact to stay out of prison."

Ms McInnes questioned her position, telling MSPs that 40% of people sentenced to severe assault or attempted murder were given community sentences so did not lose their vote.

"A blanket ban on prisoner voting is not a coherent position," Ms McInnes argued.

But Ms Goldie said the distinction is the custodial element.