Britain’s Prince Philip has been admitted to the hospital for or an exploratory operation following abdominal abnormalities, Buckingham Palace announced Thursday. The Palace told NBC News that it was a "planned operation" following medical investigations during the last week. The prince, 91, is expected to remain in the hospital for two weeks, the palace said. The announcement comes three days after Philip pulled out from a gala reception attended by Queen Elizabeth at St. James Palace after falling ill. This is his fourth trip to the hospital in 18 months. The Duke of Edinburgh, who married Queens Elizabeth in 1947, is the oldest spouse of a reigning British monarch in history.