Banning prisoners from voting in the independence referendum could breach their human rights and be open to legal challenge, a Holyrood legal adviser has warned.
The Referendum (Franchise) Bill, which specifies who can vote in the autumn 2014 poll, states: "A convicted person is legally incapable of voting in an independence referendum for the period during which the person is detained in a penal institution in pursuance of the sentence imposed on the person."
But the European Court of Human Rights previously ruled that the UK's general ban on prisoners voting in elections is incompatible with the European Convention on Human Rights (ECHR), and this could extend to the independence referendum.
The Bill is "likely" to comply with ECHR but there are "possible question marks" over its compatibility, and "the outcome of any legal challenge may be unpredictable", according to Holyrood Referendum Bill Committee adviser Professor Stephen Tierney.
The European Court previously found that prisoners' right to vote in elections does not extend to referendums. A previous challenge to the ban, in the 1975 referendum on the UK's membership of the European Economic Community (EEC), was dismissed on these grounds.
But domestic courts may not necessarily follow ECHR case law and could interpret human rights rules differently, according to Prof Tierney.
Secondly, the EEC referendum was deemed "purely consultative", rather than binding. The independence referendum may be viewed differently by a court, leaving the outcome of a challenge "open to question", he said.
"It might be argued that the law has moved on and the European Court is becoming more protective of the right to vote. Could this lead to a more 'purposive' interpretation which might catch referendums within its remit? It has been argued that it is logically incongruous that the ECHR guarantees to prisoners a right to vote in parliamentary elections but not on such an important issue as independent statehood, and on this basis that a court might read such a right.
"Challenges could be brought either by a law officer or post-enactment by way of a judicial review application. I am not persuaded that either challenge would be successful but the caveats I mention should be considered."
The Referendum Bill Committee will consider Prof Tierney's advice at its meeting in Holyrood.