The Scottish Government has put its European policy in limbo, leaving uncertainty over its role in a referendum on the proposed treaty.

The change has come as the Westminster administration is coming under intense pressure to hold a referendum on the revised version of the European constitution, now known as a treaty. Three former Labour ministers yesterday took part in the launch of a cross-party campaign for a referendum.

Prime Minister Gordon Brown has said the constitution's text has changed sufficiently for it to be supported by the UK Government without a referendum, and that Britain has won the exemptions it demanded. But he faces calls from the opposition, trade unions and around 120 of his own Labour backbenchers who say there should be a public vote on the proposals.

The Scottish National Party used to be in favour of a referendum, and was committed to campaigning against the constitution as it stood two years ago. That was because the document gave exclusive rights to Brussels institutions to control fisheries.

But while the party still argues for national control of fisheries, it has opted not to engage in the debate over the treaty. Now that it is in government, it has agreed to work within the constraints of the Common Fisheries Policy.

A senior SNP source said the party is not sure whether it wants a referendum, and if there is one, it is not sure which way it would campaign.

That is being left until after Mr Brown attends a European summit next month, which is expected to finalise the text of the treaty.

It is understood that the SNP will debate its stance on Europe and come to a new position at its annual conference, which takes place not long after the summit.

It may be forced to take a clearer position before then by the European and external relations committee of the Scottish parliament, which agreed this week that it wants to call the Europe minister, Linda Fabiani, to explain the administration's approach.

But the pressure is far greater at Westminster, where the I Want a Referendum Campaign was launched outside Parliament, with a warning that failure to give the public a say would erode democracy and increase voter cynicism.

The drive unites Eurosceptic and pro-European MPs who insist it is neither a campaign for a No vote nor a poll on whether the UK should remain in the EU.

The campaign is chaired by Derek Scott, Tony Blair's former economic adviser.

Gisela Stuart, who was Labour's appointee to the European convention that drafted the original constitution, argued: "It is not sufficient to say that because we are a parliamentary democracy there shouldn't be occasions when we do have a referendum and ask the people. It is a significant treaty which Parliament cannot amend and I think we do owe it as a matter of trust to ask the people."

Kate Hoey, a former Labour sports minister, commented: "We all feel particularly strongly about a referendum because, in all our party manifestos, it was clear that we would have a referendum.

"Probably most people who have read the detail over the summer will know that there is so little difference between the constitution and the treaty.

"We need the referendum which is why it is very important that this is an all-party campaign and will grow from strength to strength."

Senior Conservative MP David Heathcoat-Amory, a former Foreign Office minister, said: "We all have very different political opinions about the future of the country, but we all agree that people should be given a choice.

"Democracy drains away when all the decisions that ought to be made here in Parliament are made for us by people we don't elect and can't remove.

"So we are all coming together for democracy and the supreme democracy is giving people a choice about how they wish to be governed."

www.iwantareferendum.org