A total of 4,500 people waited more than nine weeks in September 2009, according to Department of Health figures. By December 2009 the number of people waiting more than nine weeks had risen to almost 16,000.

Health Minister Michael McGimpsey said health trusts had suspended waiting list work until he received clarity on how much money he would have to tackle a potential swine flu epidemic. That budget was only finalised in October.

"This further rise in waiting times is disappointing. However, the vast majority of patients continue to receive their care and treatment within a reasonable period," Mr McGimpsey said.

"It is important to bear in mind that, overall, hospital waiting times have significantly improved in recent years and are unrecognisable compared to 2005, when under direct rule patients had to endure unacceptably long waits."

He said the health service was operating under severe strain and trying to respond to increased demand with limited resources.

The minister said the increase in demand in the first nine months of 2009-10 was equal to around 35,000 additional first outpatient appointments and 9,000 more inpatient and day-case procedures.

"In the past two years alone, there has been an increase of some 20% demand for hospital services, yet our funding has only increased by 0.5% in real terms, the lowest rise in living memory," he added.