Horrified that Scotland might break up the United Kingdom by voting for independence this autumn, thousands of Northern Ireland loyalists are preparing to fight back using their favoured 17th-century battle regalia: drums, flutes, banners and orange sashes.

Loyalists, named for their allegiance to the British throne, will march in Edinburgh five days before Scotland's Sept 18 independence vote in a powerful expression of their concern at the threat to the union that currently underpins the troubled province's identity, struggling economy and divided politics.

By breaking up the United Kingdom, Scotland would force England and Northern Ireland to reassess their constitutional relationship, whose divisive details helped to fuel 30 years of bloodshed between Protestant loyalists and Catholic republicans who wanted to unite with the Irish Republic to the south.

"Scottish nationalism poses a threat to the union which is more powerful than Irish Republicanism," said Mike Nesbitt, head of the Ulster Unionist Party, the region's second largest pro-British party. "Whatever the result is, there will be some form of recalibration of the union," he added.

The anti-independence umbrella group, Better Together, has urged organisers to cancel the Edinburgh march, fearing that it could be seen as anti-Catholic and runs counter to their vision of a modern, inclusive Britain.

But the Orange Order, founded in 1795 to protect Protestant interests, insists it must stand by its Scottish brethren.

Psychological blow

A Scottish exit would be a huge psychological blow to the province's loyalists, who express their allegiance to London by lining their streets with Britain's Union Jack flags, some even painting kerbstones in its red, white and blue.

Some warned that Scottish independence would revive Irish nationalist hopes of a united Ireland, boosting support for Irish militants - largely silent since a 1998 peace deal - and raising the spectre of retaliation from pro-British militants.

"A 'Yes' vote would have tremendous repercussions for unionism," said Graham Walker, professor at Queens University Belfast, who sees fears about the UK's future as playing a role in recent tensions. "A lot of the unionist insecurity ... is influenced by perceptions that the United Kingdom is fragile itself."

Even a 'No' to independence vote could have profound consequences, with England reassessing ties with Scotland, Wales and Northern Ireland and raising question marks over tens of billions of pounds in economic subsidies.

Any changes could upset an already fragile peace between pro-Irish nationalists hit by the brief arrest of their long-time leader Gerry Adams in May and pro-British unionists furious at recent restrictions on flying the British flag.

Scotland's Orange Order, an offshoot of the Irish group, has said it hopes tens of thousands will march in Edinburgh. It would be the largest exodus of members to Scotland for decades.

But with sectarian and political divides very different in Scotland and Northern Ireland, the march may be a risky move.

"The march could actually backfire on us," said Charles Duff, 63, a Protestant shipyard worker from Inverkeithing as he prepared to march in Bathgate, 20 miles west of Edinburgh, in June. "You don't know how the people will react."

Simmering tensions

The Scottish referendum comes during one of the worst political stand-offs in Northern Ireland's power-sharing government since its establishment in the wake of the 1998 Good Friday peace deal that mostly ended sectarian violence, known as "The Troubles" that claimed the lives of more than 3,000 people.

The province's leader, First Minister Peter Robinson, last week said the institutions of power were under threat following a restriction on an Orange Order march past a Catholic estate in West Belfast.

Some of the worst rioting in years was sparked 18 months ago by a decision to limit the number of days the British flag flies in Belfast after the growing nationalist community secured its first majority on the town's city council.

Working class loyalists who fought daily with police for more than a month complained their culture was being eroded and that their communities were losing out under the terms of the peace deal, a constant refrain in politics here.

"A 'Yes' vote would be destabilising, people would find it very hard to come to terms with," said David Hume, a member of the Orange Order, who is planning to march in Edinburgh and who is concerned about increased devolution in the wake of a 'No' vote.

"The devolved powers we have are not seen to be working terribly well. And to then devolve further powers to I suppose a cauldron of descent in some ways - I don't think would be terribly workable or helpful."

Nationalists lie low

Some Unionists fear Scottish independence could revive dreams of a united Ireland, with MP Ian Paisley Jr, son of the renowned orator Ian Paisley, warning Britain's parliament in February that Scottish independence could "get the tails up of Irish Republicans".

Mainstream nationalist parties would have little to gain from an upsurge in violence and privately acknowledge that the time is probably not right to pursue a referendum on a united Ireland - a move that is allowed no more than once every seven years under the 1998 peace deal.

In the last major poll in 2011, only one-third of nationalists said they wanted a united Ireland, with many privately voicing concerns about Ireland's weaker economy and social safety net.

Ireland's Dublin government, co-guarantor of Northern Ireland's peace with London, has also been quiet, other than a line in its National Risk Assessment report that the Scottish vote could introduce an "element of instability" in Northern Ireland.

With relations light years ahead of where they were 20 years ago, following a hugely symbolic visit by Queen Elizabeth in 2011, the Irish government does not want to rock the boat.

The worst case for Dublin would be if an exit by relatively pro-European Scots tipped the balance of the UK against staying in the European Union, leading to what the Irish Risk Assessment described as "profound uncertainty into Anglo-Irish relations".

Celtic cousins

Scotland is more than just a neighbour to Northern Ireland. Millennia of migration across the narrow 20 miles (30 km) of sea has intertwined their history, culture and languages, leaving many Catholics and Protestants more loyal to their fellow believers across the sea than neighbours at home.

Many Northern Irish Protestants trace their roots to Scotland via a migration in the 16th and 17th centuries that is still seen by some Irish nationalists as an occupation.

Every year thousands of Scots travel to Northern Ireland on July 12 for marches to celebrate a 1690 victory by King William III in Ireland which cemented Protestant domination.

The parades, led by marching bands carrying 17th century style military banners, often lead to stand-offs with Catholic communities, who say they are triumphalist.

Hundreds of Northern Irish Orangemen travel in the other direction for similar, smaller parades in Scotland.

"We're not so much worried about the vote as confused," said John Keenan, 57, who runs a Union Jack flag shop in east Belfast, saying Scots were among his best customers.

Next to the Union Jack bow ties and tiaras, Keenan sells memorabilia of Glasgow Rangers football club, whose shirts are among the most potent symbols of loyalism for unionists, as Glasgow Celtic shirts are for Irish nationalism. Rivalry between the two Scottish clubs is legendary.

(VoR, Reuters)