Scotland on Sunday can reveal that activists opposed to the ban will force a vote on the floor of the conference, urging their colleagues to scrap the plan.

The rebellion is believed to have significant backing and, if successful, would amount to a serious political blow to the First Minister and Justice Secretary Kenny MacAskill. Both believe the under-21 ban would reduce binge drinking and cut crime. The plan was rejected by the Scottish Parliament two weeks ago after the Tories, Labour and the Lib Dems all united to oppose the move.

Salmond and MacAskill have vowed to plough ahead. Under their plans, the age limit of buying a drink in a pub or restaurant would remain at 18, but people would have to be 21 before being able to buy alcohol in an off- licence or a supermarket. But if they lose the conference vote, the policy will be buried for good.

Scotland on Sunday has obtained a leaked copy of the official party agenda due to be handed out to the conference, which begins in Perth on Thursday. It shows that the group Young Scots for Independence have forced the matter into the open.

In an amendment to a motion on alcohol abuse, they urge the conference to agree that "proposals to amend legislation that would increase the age of purchase for alcohol in supermarkets and off-licences will do little to tackle the real problem with Scotland's relationship with the bottle".

SNP sources said the fact the amendment had been accepted onto the party's official conference agenda was itself significant. One senior SNP figure said: "It's very interesting that they have let it go through. The point is that this under-21 ban has never been put to the party. It was just landed on people by Kenny MacAskill. I think people want to have their say on it because it isn't party policy."

Another party member said: "Alex Salmond and Kenny MacAskill have stirred up a hornet's nest. This was dreamt up as a populist policy and they have been surprised by the sheer ferocity of the opposition to it. They are in great danger of shooting themselves in the foot."

Tom French, coordinator of the Coalition Against Raising the Drinking Age in Scotland, said: "We call upon the SNP conference to avoid the temptation to simply act as loyal lap dogs to the leadership, and instead make an independent judgment based upon the evidence."

However, Shona Robison, the Public Health Minister, said: "We believe raising the off-sales purchase age would cut the level of uncontrolled, open-air and home drinking which we know can result in anti-social behaviour."

As well as being opposed by the Scottish Parliament, it emerged last week that the plan is not supported by the Scottish police, who have written to the Scottish Government warning that the move will be ineffective, and that it risked "demonising and alienating perfectly law-abiding 18 to 20-year-olds".

Labour's justice spokesman Richard Baker said: "It's time for the SNP to drop this daft policy and start working with other parties on workable solutions to Scotland's alcohol problems."