The Scottish Football Association, as well as a growing body of fans, fear the one-off proposal could herald the end of Scotland, Northern Ireland, England and Wales keeping their privileged status as independent footballing nations.

The row intensified after Jim Murphy, the Scottish Secretary, announced he had secured "categorical assurances" from the sport's governing body, Fifa, that a joint UK team would not undermine Scotland's autonomy in interna-tional football.

He said: "The proposal is for a UK team as a one-off in the Olympics in London in 2012 for players under the age of 23."

Fifa confirmed yesterday that Jerome Valcke, its secretary- general, had welcomed the idea of a Team GB in 2012 and said it would not affect home nations' membership.

However, it declined to say if and when this would be ratified by all 208 member associations.

A Great Britain football team has entered the Olympics before, winning gold medals in 1900, 1908 and 1912. But the idea of a unified team for 2012 has not gone down well. An SFA spokesman said: "While we would welcome any statement from Fifa, we have to be clear that Fifa is an organisation made up of its members – and it is their views that are so important in this issue. At some point, there is a real danger that a precedent of a Team GB will come back and threaten our status as a separate nation."

Fans also dismissed the Fifa pledge as meaningless, saying that statements from executive members did not offer binding guarantees.

Tam Ferry, a spokesman for No Team GB.com, said: "The last time I looked, Fifa was a democratic organisation.This sort of special dispensation would depend on a vote by the full membership of Fifa.

"It's all just part of Gordon Brown's Britishness campaign and efforts to win more medals. They are obviously pushing Valcke for a guarantee, but he's in no position to give one."

Jim Sibbald, secretary of the West of Scotland branch of the Tartan Army, said: "I am totally against any Scots playing in a Great British team. There are no guarantees that this would be a one-off. Over the years, Fifa has been desperate to get us to play as one nation, and this would give them a foot in the door."

Mr Murphy also faced a political backlash, with Alex Salmond, the First Minister, accusing him of "opening a Pandora's Box".

He said: "We know that if we go in for this, then it will be used by other countries at future Fifa assemblies to question the status of the home nations in international football.

"The idea of doing it has got nothing to do with football, and everything to do with politics. The first thing Gordon Brown said at the Beijing Olympics was that he wanted a British team in the Olympics next time. This is Jim Murphy following Gordon Brown's orders. I think he should get a few more air miles under his belt as Scottish Secretary before he ventures where angels might fear to tread."

He went on: "We've got a privileged position in international football. We are one of the very few countries who are able to be an independent football country without being an independent country. And the idea of sacrificing or jeopardising that status for the sake of the participation of a few players in an under-23 tournament as a one-off is absolute madness."

Tavish Scott, leader of the Scottish Liberal Democrats, said: "Instead of getting into arguments he won't win with Scottish fans, the Scottish Secretary should be following my lead and promoting Scotland as the host for the European Football Championships."