The Crown body responsible for flag protocol in Northern Ireland has said that flying the Union flag 365 days a year is not a British tradition.
Displaying the flag on a permanent basis is more in line with attitudes in other countries such as the United States, according to the College of Arms.
In a statement to the Belfast Telegraph, the body, which is a branch of the Royal Household, said flying the flag permanently meant the designated days – including the Queen's birthday – lost their distinctiveness entirely.
Its official advice appears to undermine unionist claims that their Britishness would be eroded if the flag is not flown every day.
The Alliance Party claimed it vindicated their decision to support a vote limiting the flying of the flag at Belfast's City Hall.
Its group leader on Belfast City Council, Maire Hendron, said: "Not only does the Alliance Party's position on designated days reflect the Good Friday Agreement, Northern Ireland's constitutional position and the fact that Belfast is a diverse community, but it is also in accordance with the views expressed by the College of Arms.
"How can unionists claim designated days is an erosion of their Britishness in the face of such an inherently British institution?" According to the Royal website, the flying of the Union flag on public buildings is decided by the Department for Culture, Media and Sport at The Queen's command.
The department lists the designated days, but adds individual departments can decide whether to fly the flag outside of these dates.
"We encourage everyone to follow this example, to champion the Union flag and fly it daily as a symbol of national pride," it adds.
Yet the College of Arms, which has responsibility for flags and other heraldic matters for Northern Ireland, England and Wales, said it was British custom that it was only flown on the set dates.
"The practice of flying it all year round is relatively new in the United Kingdom and perhaps more in accordance with attitudes to flags seen in other countries such as the United States than with traditional attitudes here," a spokesman said.
"For one thing, if it is flown all year round, the [designated days] entirely lose their distinctiveness."
However, unionists rejected the comments. A DUP spokesman said the practice of flying the flag 365 days a year was increasing.
"The comments from the college itself note the practice of flying the Union flag on a year round basis is not new and is not without precedent," he said.
"Indeed they tacitly acknowledge that the practice has been increasing in recent years."
He said a paper produced by the previous Westminster government encouraged flying the Union flag on a more frequent basis.
"The DUP believes it is entirely appropriate for major civic buildings such as Belfast City Hall to fly the flag on a year round basis," the spokesman added.
A UUP spokesman referred this newspaper to the Department of Culture website and its remarks on flying the flag daily.
Questions and Answers
Q What exactly is the College of Arms? A It is a branch of the Royal household and is the official repository of the coats of arms and pedigrees of English, Welsh, Northern Irish and Commonwealth families and their descendants. It also has responsibility for flags and other heraldic matters in England, Wales and Northern Ireland and is charged with providing information and guidance on flag flying and protocols.
Q So what does it say about the controversy in Belfast? A The College of Arms lists the designated days when the Union flag should fly in 2013, adding that there is no requirement on any public body to fly it at other times.
Q What do other official bodies say? A The British Monarchy's official website says the flag is flown on public buildings on the designated days, adding that this is decided by the Department for Culture, Media and Sport (DCMS) at The Queen"s command. However, the DCMS website points out that departments can decide whether to fly the flag outside these days. It encourages people to fly the flag daily as a symbol of national pride.
Q What are the designated days in 2013? A The first is on January 9, marking the Duchess of Cambridge's birthday, followed by the Countess of Wessex's birthday on January 20. Other dates include Commonwealth Day, Coronation Day, The Queen's actual and official birthdays, Remembrance Day and on the days of the State Opening and prorogation of Parliament.