MSPs at Holyrood will vote today on whether short-term prisoners should be allowed to take part in the historic ballot.
Ahead of that, the Howard League for Penal Reform in Scotland is urging them to give the vote to inmates on shorter sentences.
Chair John Scott QC argued that imposing a blanket ban sent prisoners “the message that they are not part of Scottish society”.
Although the Scottish Government has repeatedly stated its opposition to giving prisoners the vote, Liberal Democrat Alison McInnes will call on MSPs to allow those serving shorter sentences to take part in the independence referendum.
Ms McInnes has put forward two amendments to the Scottish Independence Referendum (Franchise) Bill, which could enable either those serving a sentence of up to six months or those sentenced to up to four years to vote in the referendum.
The proposals have already been rejected by MSPs on the Holyrood committee set up scrutinise the referendum legislation.
But Mr Scott said: “Scotland looks increasingly isolated in seeking to enforce a blanket ban on prisoner voting.”
He claimed only four other countries in Europe denied all convicted prisoners the right to vote, adding these were all former communist dictatorships.
Mr Scott argued: “Many of those serving a sentence in Scotland’s prisons are drawn from our most deprived and disadvantaged communities.
“Denying all of them without exception the right to a say in the future of the country they live in is to send them the message that they are not part of Scottish society. This is no way to encourage offenders to recognise their responsibilities as citizens.
“As the Bill stands, whether you are in prison for a matter of days or weeks, or for several years, you will not have the right to vote in the referendum.”
He added: “If the blanket ban is carried, we hope MSPs will take time to remember when they go to vote on September 18 2014 that some of their fellow citizens will have been prevented by an Act of the Scottish Parliament from exercising the same choice over their future.”
Ms McInnes said the Howard League had “put forward a convincing case to allow short term prisoners the vote in the independence referendum”.
She added: “Backed up by international precedent which shows only four countries maintain a blanket ban, this is the first opportunity any part of the UK has had in recent years to take a considered view on prisoner voting.”
Ms McInnes urged MSPs to back her amendments so that “Scotland could join the list of progressive European countries who have taken this responsible, reasonable and proportionate step”.
That approach was also backed by union leaders at the Scottish Trades Union Congress, who described it as “an issue of social justice, equality and citizenship”.
General secretary Grahame Smith said: “The franchise for the independence referendum will be agreed in Scotland, by the Scottish Parliament.
“This is the last chance for the Scottish Parliament to reject a blanket ban on prisoners being able to vote in the independence referendum.
“The Scottish Government’s intention to press ahead with a complete ban on prisoner voting, despite the concerns raised by so many organisations is extremely disappointing. This places Scotland in a tiny minority amongst other European countries.”
A Scottish Government spokeswoman said: “The Scottish Government does not agree that convicted prisoners should be able to vote while they are in prison.”