The measures, to be voted on by MSPs on Tuesday as part of the Victims, Witnesses, and Justice Reform (Scotland) Bill, would expand the scope of sections 274 and 275 of the Criminal Procedure (Scotland) Act 1995. These provisions regulate the admissibility of evidence relating to the sexual history or character of complainers.

Ms Constance has proposed applying the protections to docket witnesses or complainers – individuals not named in charges but whose testimony may corroborate the evidence of the cited victim – and to witnesses or complainers who have died. She said the amendments were intended to “close a gap” in the legislation.

But Simon Brown, president of the Scottish Criminal Bar Association, said the changes could seriously undermine defendants’ rights. “The extension of 274/275 to docket and deceased witnesses is presented as just closing a loophole, but this is far from the case,” he said. 

“The difficulties posed to the defence by the way the courts currently interpret 274/275 have already been well documented, with the defence bar universally agreeing that the pendulum has swung too far in the restrictions it places on lines of cross-examination.

“But there are particular difficulties posed by its extension to docket and deceased witnesses.”

He added: “A docket witness is a witness giving evidence to a docket on an indictment rather than an actual charge. This means their evidence is in support of other charges or witnesses, but will not lead to a conviction for the incident involving that witness.

“A common example would be where an accused was charged with rape, and to provide corroboration of that the Crown would add a docket saying it intended to lead evidence of an earlier incident where perhaps he’d been charged with rape but convicted of sexual assault.

“I can foresee particular problems where the docketed verdict was reached by a jury pre-274/275 who had heard evidence that the current jury may not be allowed to hear.”

A Scottish government spokesperson said: “Everyone has the right to a fair trial and to appeal against a conviction or sentence. The amendments included in the Victims, Witnesses and Justice Reform Bill will extend the same protections already provided for victims in sexual offence cases to also include witnesses in certain circumstances.”