Scottish unionists fear that a pro-Union Orange march through Edinburgh may encourage Catholic voters of Irish background to vote for independence.
Last night Rev Mervyn Gibson, the assistant grand master of the Grand Orange Lodge of Ireland, confirmed that thousands of members from Northern Ireland will be travelling to Scotland for the demonstration.
"Representatives from Grand Lodge, county lodges and individual lodges are making their own way over. You could certainly have thousands going over between bands and brethren."
He added: "Scottish brethren stood beside us during the Troubles and we need to stand by them when their place in the Union is under threat politically."
Edward Stevenson, the Irish grand master, will be addressing a rally along with the Scottish and English grand masters Henry Dunbar and Ron Bather.
The fears over the march come as support for Scottish independence rose eight points in a month, according to a new poll.
The No camp is now six points ahead of the Yes campaign, down from 14 points in mid-August and 22 points early last month, excluding undecided voters.
The Better Together group, led by former chancellor Alistair Darling in campaigning against independence, is at pains to distance itself from Orangeism, despite their shared support for the Union. In response, the Order has set up its own British Together campaign which is "fervently opposed" to Scotland breaking from the UK.
"The Orange Order have nothing to do with us," a Better Together spokesman said yesterday.
Earlier this year, the organisation said the Order "isn't part of our campaign and never will be".
"The best way for people who believe that we are stronger and better together as part of the UK to get involved is by speaking to undecided voters, not marching in the streets," he added.
Better Together fears Orange rhetoric could alienate swing vote Catholics in the days before the vote, but Rev Gibson disagrees.
"I have heard that argument," he said. "The Orange Order is a major part of Scottish life and is entitled to demonstrate its view. I hope that it galvanises people to vote No.
"If people are only voting Yes to be against the Orange Order then that would be very shallow support to begin with."
However, Better Together planners see Catholic working-class voters of Irish extraction in the west of Scotland as a key group whose support they must retain if they are to win the referendum. In 2011, many in this group switched from Labour and helped to deliver the Scottish National Party an overall majority in the Scottish parliamentary elections.
"That is where I think the referendum will actually be fought out. The danger of an Orange Order march in the few days before the referendum is that it could unsettle these voters," said one Better Together source.
"Religion is much less significant here than Northern Ireland but its legacy can be seen in the Orange Order affinity with the Union and the affinity of some Celtic supporters with nationalism. We have identified people who see themselves as of broadly Irish Catholic background as a critical group in these last stages of the campaign."
Rev Gibson dismissed fears that the march could prove counterproductive: "There will be no trouble on the 13th. This is purely a march to support the union and to encourage people to vote No."