The Scottish Government's controversial proposals to ban under-21s from buying alcohol in shops will face a challenge from within the SNP this week.

A motion opposed to the ban, which was defeated in parliament two weeks ago, will be put forward and debated by SNP youth members at the party conference, which begins on Thursday.

The Young Scots for Independence group has lodged an amendment to a motion welcoming the government's consultation on alcohol, speaking out against the off-sales policy.

Critics have highlighted the anomalies that would be created for 18 to 21-year-olds who could vote, drive, serve their country and work in off sales but will not be able to purchase alcohol from the same premises.

The motion states: "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."

The amendment ends with a call "to look more closely at enforcing current legislation that would prosecute those agents' supplying alcohol to our young Scots".

The debate will give party members the chance to air any concerns with the plan in front of the First Minister, Alex Salmond, and the Cabinet.

A vote against the ban at the conference, which takes place in Perth, could spell the end of the controversial policy. It has still to be finalised after a consultation, and has yet to go before the Scottish Parliament.

However, party leaders continued to speak in favour of the proposal yesterday, which was trialled in shops in Armadale and Stenhousemuir.

The SNP has criticised the opposition parties for, so far, failing to contribute their own ideas to the consultation, while attacking the proposal.

Mr Salmond insisted yesterday the ban had support. He said: "It is very popular with people in Scotland in the areas where it has been trialled. It seems to be having an effect and we should look at it favourably.

"It is a serious problem and it requires serious action."

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

Earlier this month, Labour, Conservatives and Liberal Democrat MSPs joined forces in the Scottish Parliament to defeat the government over a motion on the plan.

On Saturday, the Liberal Democrats, at their party conference, called on the SNP to drop the plan amid growing opposition from opposition parties, unions and student groups.

Party leader Tavish Scott said: "Tens of thousands of the brightest students in the UK are thinking of coming to Scotland next year. So if the First Minister is seriously thinking of converting Scotland's universities into a network of temperance colleges he should say so.

"Students need to know before they sign up to four-year courses.

"If those students turn away, it will be Scotland's loss. Let's have a better plan.

"Respect the votes of parliament. Drop the alcohol plans."