Twelve academics have entered a row over freedom of speech in the independence debate.

The group warns against trying to "muffle dissent" following claims that Scottish Government minister Shona Robison questioned the impartiality of a Dundee University historian.

Professor Chris Watley is involved in the "Five Million Questions" project to investigate issues raised by the referendum. But he also spoke at a Better Together event on keeping Scotland in the UK, prompting Ms Robison's intervention.

The academics' letter is signed by people from universities across the UK, including Oxford, London, St Andrews and Edinburgh.

Ms Robison's approach raises questions about academic integrity and freedom of speech, the letter states.

"First, it is absurd to claim that only those who declare no view on independence can be trusted to treat the data and the arguments honestly," the academics claim.

"On most moral and political issues most of us have commitments - and rightly so, since such things are often too important for the luxury of fence-sitting. The challenge is to achieve, not neutrality, but fairness to our opponents."

The Scottish Government must avoid looking as if it acts unfairly to opponents, it warns.

"Whether true or not, the perception alone is sufficient to muffle dissent among those whose institutions depend on public funding - not only academics, but others too," the letter continues.

"So if the Scottish government wishes to avoid justifiable charges of the partisan abuse of its public powers, it should instruct its ministers to be scrupulous in upholding the freedom of academics and others to express their political opinions."

Signatories include Hew Strachan of Oxford University, Andrew O'Hagan of King's College London, Jim Gallagher of Glasgow University and David Fergusson of Edinburgh University.

A spokesman for Better Together said: "This is a significant intervention from academics, not just here in Scotland but across the whole of the UK. It is heartening that so many of our country's top academics are not willing to be bullied into keeping quiet by the SNP. Shona Robison has yet to apologise for her unacceptable behaviour, but maybe this letter will prompt the SNP minister to think again."

First Minister Alex Salmond was challenged by Conservative leader Ruth Davidson to condemn any intimidation during exchanges at Holyrood on Thursday.

He said: "The accusation is total and utter nonsense. The words 'intimidation' and 'Shona Robison' don't sit easily together."

A spokesman for the First Minister said: "The Scottish Government welcomes all voices and views to the debate on Scotland's future and, as the First Minister made clear last week, endorses both Professor Watley's leadership of the Five Million Questions project and his right to be involved in the No campaign.

"It is absolutely right that academics, or anyone who wants to contribute to the debate, are encouraged and feel free to express their views. The point, which is touched on in the letter from the academics, is that an impartial academic exercise, like the Five Million Questions project, is not itself allied with one or other side of the referendum debate.

"Scottish Government ministers have shown themselves to be strong supporters of the Dundee University project, participating in several of its events."