Waiting time targets for Northern Ireland's hospital emergency departments were breached more than 700 times between April and June, the Department of Health revealed.
The target states that no patient should wait longer than 12 hours to be treated, discharged or admitted.
Most of the 739 cases were at the Royal Victoria Hospital in Belfast.
A report from the department said: "The ministerial target for emergency care waiting times has not been achieved during any of the last three months (April - June 2014) as one or more components of the target were not met."
It said performance declined notably at the Royal Victoria Hospital.
The Royal includes Northern Ireland's regional trauma centre which takes serious cases from across the region and deals with the biggest number of emergency department patients.
During April, 67 patients waited more than 12 hours in the Royal's emergency department, but during June 190 cases breached the target.
Prior to the period covered by these statistics, a major incident was called at the department in January when too many people were left waiting on trolleys.
Not enough medics were available at times in the Royal's emergency department to properly treat patients, inspectors have found.
About 100 staff who were interviewed raised concerns about staffing levels, bullying, intolerable pressure and a dysfunctional healthcare system.
Colm Donaghy, chief executive of the Belfast Health and Social Care Trust, resigned earlier this year.
The target set by Health Minister Edwin Poots for emergency care waiting times in 2014/15 states that 95% of patients attending emergency departments should be either treated and discharged home or admitted to hospital within four hours of their arrival at the department and no patient attending any emergency department should wait longer than 12 hours to be treated and discharged or admitted.