The UUP leader is disappointed that the Secretary of State will not add the birthday of Prince George to the list of designated days for flying the Union flag at Stormont.
The flying of the flag at Parliament Buildings is governed by the Flags (NI) Order 2000, which permits the flag to be hoisted 15 days a year.
The designated days celebrate the birthdays of royals, St Patrick's Day and other significant dates.
It was last updated in 2002, following the deaths of the Queen Mother and Princess Margaret.
But the Department of Culture, Media and Sport is responsible for publishing the list.
UUP leader Mike Nesbitt said he has told the Secretary of State Theresa Villiers that the designated days list in Northern Ireland is "out of step with the list operated by the DCMS in London".