Controversial SNP plans to ban off-licence sales of alcohol to under-21s have received a new blow after a senior statistician attacked the way in which data from localised trials of the restriction were being interpreted by government ministers.

Professor Sheila Bird, vice-president of the Royal Statistical Society, told the Scotsman that figures showing a drop in anti-social behaviour in areas where police had enforced the measure were “hugely statistically insignificant.”

“It's disappointing that in such a potentially important intervention as this, there are more questions than answers,” said Professor Bird.

"There have been big changes in some outcomes, but the studies do not allow you to attribute them specifically to the underage ban.

"With something as important as this, you need a proper study."

Three Scottish police forces have been trialling "Stop the Supply" proposals to prevent shops from selling alcohol to under-21s in Armadale, Stenhousemuir, Larbert and Cupar.

Other measures being tested alongside the sales restrictions include "sting" operations using underage alcohol buyers as police informers, marking bottles to identify shops selling alcohol to underaged clients, an information service on underage drinking for shopkeepers, and increased targeted police patrols.

The results of the pilot schemes have been heralded as an overwhelming success by SNP ministers, who introduced the proposals as part of a flagship public order and alcohol misuse policy consultation in June.

Health Minister Shona Robison said: “The dramatic results from ‘Stop the Supply’ show what can be achieved when communities take bold steps to tackle alcohol misuse among young people.

“Taken together, we believe these measures could help bring about the long-term cultural shift needed to rebalance Scotland's relationship with alcohol.”

Trumpeting police figures that claim incidents of anti-social behaviour fell by 40 per cent in the first three months of the trial, an SNP press release claimed that “an over-21s off-sales policy could help cut crime and antisocial behaviour if extended nationwide.”

However, Professor Bird has attacked the SNP’s use of the preliminary figures. She singled out the claim that one trial had seen a 60 percent drop in serious assaults as “naughty”, pointing out that it referred to a fall from five incidents to two.

“The way the results are being reported, it may not even be spin; it may be naivety,” she added, stating that properly controlled studies of the under-21 ban would have to be implemented before its effectiveness could be determined.

The proposals have generated considerable controversy, soliciting criticism from civil rights organisations, student groups, business lobbies and politicians. A petition against the move has gained over 10,000 signatures.

Drinks producers have spoken out against the SNP initiative, condemning the attempts to “demonise” alcohol and drinking.

“Problems of alcohol misuse in Scotland will not be solved by turning alcohol into a social taboo and demonising drink,” said David Poley, chief executive of the Portman Group, the drinks industry lobbying group.

“There is a considerable risk that this would actually increase the appeal of alcohol to young people in particular. The myth is that levels of drinking and alcohol misuse are worse in Scotland than elsewhere.

“In reality, Scots drink less than people in the rest of Britain and are no more likely to be drinking harmfully.”

His views were echoed by Gavin Partington, of the Wine and Spirit Trade Association: “The government has said that it doesn't want to demonise alcohol, but if you take all these proposals together, this is what they will do.”

In addition to the under-21 ban, draft legislation could also include proposals to set a minimum price for a unit of alcohol.

Police officials involved in the trials have been cautiously positive regarding their outcome. Chief Inspector Bob Beaton, who led the crackdowns in Stenhousemuir and Larbert, argued that the anecdotal evidence was encouraging.

“It's difficult to separate the different strands to say which have been most successful. It does appear this is working, but there have been a number of initiatives and the under-21 alcohol ban is just one.”