Earlier, the Princess apologised for "any distress which may have been caused" by her visit on Sunday with Princes William and Harry to see The Devil's Own, accused by some of glamorising the IRA.
She was also criticised for taking 12-year-old Harry into a 15-certificate film, after persuading cinema staff at the Kensington Odeon, west London, to allow him in. Prince William was 15 on Saturday, the day before the visit.
The film has been condemned as pro-IRA even by its stars, Brad Pitt and Harrison Ford. Pitt reportedly described it as "the most irresponsible bit of film-making - if you can even call it that - that I've ever seen."
Moreover, the cinema trip came just six days after the murder by the IRA of two RUC officers in Co Armagh.
The film itself has opened to mixed reviews. Alexander Walker, of the London Evening Standard, claimed it lacked any insight into the tragedy of Northern Ireland, but instead fostered "sympathy for romanticised terrorism ... by allowing Brad Pitt to get as many sexy kicks as he can out of ending people's lives".
While the Princess was on her way to a private viewing in Manhattan of 75 of her dresses, which are to be auctioned for charity, she issued her statement, in which said she had been "unaware" of the film's content.
The Ulster Unionist Party led the criticism of the Princess. East Londonderry MP William Ross said: "There have been a number of films in recent years about the IRA which give a twisted view about what is happening in Her Majesty's realm, and I think she was unwise."
Ron Hanlon, marketing director for Odeon cinemas, said there would be an internal investigation into the presence of a 12-year-old at the film, after which he expected talks to take place with the local licensing authority, Kensington and Chelsea council.
A spokeswoman for the Princess declined to say whether she had been in contact with Prince Charles over the row. Michael StreeterReuse content