Police estimates that a third more drink-drivers will be caught when the alcohol limit is reduced next month is not supported by international evidence, according to Justice Secretary Kenny MacAskill.

The drink-driving limit will be reduced from 80mg to 50mg per 100ml of blood on December 5, backed by a widespread public awareness campaign to alert Christmas revellers.

Police estimate they are likely to catch around a third more drink-drivers, Chief Superintendent Iain Murray told MSPs last week.

But Mr MacAskill told Holyrood's Justice Committee today that was an "extrapolation from the raw statistics", and evidence from the Republic of Ireland shows convictions fell after they reduced the limit in 2011.

He said: "Although the stats provided by Chief Superintendent Murray are correct, equally the practical delivery of this from Ireland (shows) it went down because it drove home a message to people: don't drink and drive."

Conservative MSP Margaret Mitchell said: "So you're really dismissing this third that was given in evidence?"

Mr MacAskill said: "I'm not dismissing it. I'm referring to the clear statistical parallel from the Republic of Ireland when they did the same change that we propose to do.

"Their perceived wisdom would have been that more people would be caught because more people would have been 50mg and beyond, but in reality less people were caught because more people adhered to the law and didn't risk it."

He added: "What Chief Superintendent Murray has tried to do is simply tried to work out those who were between 50mg and 80mg and therefore more likely to be caught.

"So its extrapolation from the raw statistics.

"But in Ireland in 2007 there were 19,848 statistics, in 2010 there were 12,602, in 2011 when the lower limit was enforced from October 28 it had gone down to 10,575 and from October 28 2011 to October 27 2012 the numbers convicted fell to 9,771."

Last week, Mr Murray told the committee: "We estimate that we are likely to catch around a third more drink-drivers than we do at the moment in the initial phase.

"Research shows that drink-driving and alcohol counts across the board tend to drop following the introduction of lower limits. My hope, therefore, would be that the public would learn.

"A quite significant campaign will be ratcheted up through November into December to make people aware of the implications. We have been doing that through the drink-drive initiatives of last winter and summer. When we breathalyse people who are over the proposed limit but under the current limit, we make them aware of the situation.

"The worst-case scenario could be that as many as a third more drivers will be caught, but I would like to think that it will be less than that."