By Ben Davies BBC News Online political staff

If Tony Blair calls a referendum on UK entry into the single currency the ensuing campaign is likely to make for some very strange bedfellows.

Broadly speaking, Conservative MPs will oppose joining and Labour members will campaign for the euro.

But within each party there are plenty of dissenters.

The likes of Tory grandee Michael Heseltine and former chancellor Ken Clarke will be on the same side as a Labour prime minister.

Left and Thatcher

People on the left of Labour, meanwhile, will find themselves siding with Lady Thatcher by backing a 'No' vote.

Michael Foot campaigned against the Common Market

It all makes life difficult for the political strategists - and according to one leading Tory, his party will choose to take a back seat in the eventual campaign.

The claims of strategy chief Dominic Cummings have been rejected by party chairman David Davis, but the affair highlights the uncertainty over which way to go when it comes to the euro referendum.

Mr Blair has recently appeared to raise the stakes by hinting at the prospect of a referendum by next year.

In his own party, rival groups are setting up both to push for and oppose joining the euro.

The prime minister meanwhile has been repeatedly criticised for failing to give an effective lead to the yes camp.

In the days of the last referendum the likes of Labour's Peter Shore and Tory right-winger Enoch Powell urged the British public to back withdrawal from the then Common Market.

Margaret Thatcher and the right of the Labour Party meanwhile were at the front of the campaign for the UK to stay in.

Big issue

Ted Heath took the UK into Europe

A referendum on the euro will see usual political alliances put to one side while the nation concentrates on what most agree will be the biggest political decision of a generation.

During his second unsuccessful campaign to become Tory leader in the wake of the resignation of William Hague, Ken Clarke sought to play down the single currency as a party political issue.

Yet Mr Blair looks set to front the campaign for euro-entry while the Tory leadership will campaign to keep the pound.

According to the No campaign they intend to "depoliticise" their fight to keep the pound.

Keep the pound

They say the Tories will continue to oppose joining the euro in Parliament, and the wider battle will see business leaders and showbiz personalities wheeled out to urge a 'No' vote in the referendum.

Peter Shore was an opponent of European federalism

It is all a far cry from the days of William Hague who tried to turn around Tory fortunes by waging a 'Keep the Pound' campaign ahead of the 2001 election.

The outcome of that poll saw the Conservative leader consigned to political oblivion and his successor, Iain Duncan Smith, seems to have learned the lesson.

He will be hoping that the opinion polls prove correct and that majority of British voters opt to keep the pound.

But Mr Duncan Smith will know that if the prime minister calls a referendum he will do so in the belief that he can win it.