The lord advocate gave her opinions in a letter to an MP
Scotland's top prosecutor has said the case has not been made for extending the length of time terror suspects can be detained without charge to 42 days.
BBC Scotland has learnt that Lord Advocate Elish Angiolini gave her opinion in a letter to the Liberal Democrat MP Alistair Carmichael.
She said the change from the current 28 days was not supported by "prosecution experience to date".
The House of Commons is due to vote on the issue on Wednesday.
The opposition parties argue the proposed 42-day pre-charge detention limit will infringe civil liberties but ministers argue it is necessary to deal with increasingly complex terror plots.
UK Home Secretary Jacqui Smith has offered a number of concessions in an attempt to win over up to 50 rebel Labour MPs.
Ms Angiolini's wrote to Mr Carmichael: "While there has been a limited number of cases in Scotland which were investigated in terms of the Terrorism Act 2000, I am not aware of any case where an extension of the period beyond 28 days would have been required.
"I therefore share the view of the DPP (Director of public prosecutions) Sir Ken MacDonald and the former Attorney General, Lord Goldsmith, that the requirement for an extension to the current 28 day is not supported by prosecution experience to date."
The lord advocate, in a highly significant intervention, has said that she is unaware, over the past eight years, of any single case where that extra power would have been needed
Fergus Ewing Scottish Government minister
Mr Carmichael, the MP for Orkney and Shetland, said: "I cannot think of a precedent for an intervention of this kind by the lord advocate.
"It is incredible that the home secretary insists on continuing in the face of opposition, now from the lord advocate and the director of public prosecutions.
"We must also bear in mind that Elish Angiolini is a career prosecutor and not a politician, unlike her predecessors."
Scottish Government minister Fergus Ewing said he shared her doubts about the need for the change.
He said: "The lord advocate, in a highly significant intervention, has said that she is unaware, over the past eight years, of any single case where that extra power would have been needed."
Ms Angiolini's predecessor, Colin Boyd, told BBC Radio Scotland: "Being prepared is one thing.
"But putting in place a law which allows you to hold suspects for such a lengthy period of time should only be done where you have some evidence that it is necessary."
Lord Boyd added that he was concerned about the impact on Muslims and other ethnic minorities, as the laws appeared to be increasingly aimed at them and "draconian" in their effect.
"We don't want to have a situation where the law is a recruiting-sergeant for terrorism."
Former Lord Adcovate Lord Fraser backs Elish Angiolini
Former Lord Advocate, Lord Fraser of Carmylie, said Ms Angiolini had made a courageous stand on the issue.
He said there was "absolutely no justification" for increasing the number of days a terror suspect could be detained without charge.
"Actually the Crown Prosecution Service has found that it can operate quite satisfactorily within a 14-day period of detention," he said.
"So even 28 days is not justified, 42 certainly is not."
Baroness Helena Kennedy, a QC and Labour peer, said she believed Ms Angiolini's comments would be seen as significant by MPs preparing to vote on the issue at Westminster.
Lady Kennedy added: "I am sure she had to weigh up very carefully how she answered that letter, and I am sure she had in mind that it would create a wave, but it isn't actually going against what most senior lawyers are saying.
"I am currently involved in these sorts of cases, and have been for the last couple of years, and I too take the view that you just do not sacrifice liberty and the protections that there are for the accused, ithout taking great care that it is the only thing open to you.
"This isn't necessary at this moment - there are ways in which you can deal with an emergency which wouldn't require having this on the back burner.
"I remember when we had the terrible atrocity of the Omagh bombing. Parliament was recalled and within 24 hours we had legislation on the books."
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