Lord Turnbull, a former chairman of the Scottish Sentencing Council, told the Scottish Parliament’s Justice Committee that because a discount of one third is applied to early guilty pleas, sentences of 18 months could be brought within range of the presumption, resulting in such offenders getting community sentences, as reported by Scottish Legal News in February.

He said, however, that sheriffs may take this quirk into account as they did when the original three-month presumption was brought in, a practice called “up-tariffing”.

Lord Turnbull said: “We’ll have to wait and see what individual sentencers decide in that situation.

“But I would have thought that if the sentencer starts off with the conclusion that a sentence in the region of 15 to 18 months is the only appropriate sentence, he’s quite unlikely to change his mind.

“Yes, he’ll think again, but is he likely to come to a different conclusion? I’m not so sure.”

Professor Cyrus Tata, a criminal law academic from Strathclyde University, expressed scepticism about the Scottish government’s claim that the presumption against three-month sentences had reduced reoffending, describing it as “dodgy” as it was based on a crude comparison between different types of offender.

Conservative MSP Liam Kerr said: “Victims were already disgusted at the idea of scrapping sentences of less than a year.

“The fact this seems to be increasing to 18 months will compound a feeling that they’re being let down by a soft touch justice system under the SNP.

“It’s also very concerning to hear that violent criminals who attack police, paramedics and firefighters could be let off the hook. The SNP needs to think again on this.

“It’s vital the justice system keeps ordinary people safe, punishes crime and offers sufficient deterrent to those considering crime.

“The SNP’s proposals fail on every single one of those counts.”

A spokesman for Justice Secretary Humza Yousaf said: “The presumption is not a ban – prison will always be the right place for the most serious offenders and those who pose a risk to the public.

“The presumption will only apply in cases where a court has reached a view that a sentence of 12 months or less may be appropriate having taken into account all relevant factors.”