The Supreme Court of Canada has ordered the appeal courts in Alberta, British Columbia and Quebec to reconsider four old drunk driving cases in light of a new precedent-setting ruling it recently issued.

The appeal courts all ruled that the four accused should face new trials after being acquitted under the so-called Carter defence, which allowed an accused to present evidence of alcohol consumption that could challenge a breathalyzer reading.

The four were charged before the federal government killed the Carter defence in July 2008 with amendments to the Criminal Code.

Their lawyers argued that the amendments could not be applied retroactively and that their original acquittals should be upheld.

In a ruling earlier this month, the Supreme Court restored a drunk-driving acquittal against a man who used the Carter defence before the law was changed, ruling that retroactive application of the new rules breached his rights.

In an unusual move, the Supreme Court has referred the four new cases back to the appeal courts and ordered them to resolve the matters in accordance with this ruling.