STORMONT raised the Union flag yesterday to mark Prince Andrew's 60th birthday despite the British government saying it is no longer necessary.
The Duke of York's birthday is one of the 20 designated days in the year for flying the Union flag on UK government buildings.
But the British government had advised that displaying the flag yesterday was "no longer a requirement" following his decision to step back from public duties.
His decision followed severe criticism over his past friendship with convicted sex offender Jeffrey Epstein.
The Department for Digital, Culture, Media and Sport (DCMS) said: "Following the decision by the Duke of York to step back from public duties for the foreseeable future, there is no longer a requirement for UK government buildings to fly the Union flag on Wednesday February 19."
The date remains listed as a designated day on the DCMS website.
Stormont's Parliament Buildings displayed the Union flag yesterday.
An assembly spokeswoman said the Assembly Commission "has agreed that Parliament Buildings flies flags on the designated days" as determined by DCMS.
Several councils in Northern Ireland also continued to fly the Union flag on Duke of York's birthday.
Belfast City Council and Armagh City, Banbridge and Craigavon council said they fly the flag on designated days as defined by DCMS, while Lisburn and Castlereagh council said it follows designated days identified by Stormont's Department for Communities.
Ards and North Down said it has seven flags which fly on council properties throughout the year and one which flies on designated days.
The council had previously said it would not be flying the flag, but changed its position. It said original decision had been taken by council officers, and had been made in error.
Antrim and Newtownabbey council and Mid and East Antrim council said they fly the Union flag every day of the year.
Causeway Coast and Glens council said each of its former council areas before they merged "follows the policy previously agreed in that area", and "in relation to designated days council take the lead from Stormont".
Derry and Strabane, Mid Ulster, and Newry, Mourne and Down councils said they have 'no flags' policies across their buildings. Fermanagh and Omagh council it was not flying the Union flag yesterday in line with its policy.
Various other government buildings in Northern Ireland are required by law to fly the Union flag on a list of designated days, including February 19.
Northern Ireland is the only part of the UK where the designated days for some government buildings is enshrined in law.
The 'New Decade, New Approach' deal to restore devolution at Stormont included a commitment to bring this list into line with DCMS guidance – increasing the number of designated days by three.
It emerged in 2018 that the north was losing Europe Day as one of its designated days due to Brexit.
