New official stats have shown a third of urgent cancer referrals in Northern Ireland wait more than two months for their first appointment.
The figure is for December 2017, and shows a decrease - from 68.7% to 66.4% - for the same period in 2016.
From April 2017, the Department's target for cancer treatment is for 95% of patients urgently referred with a suspect cancer to begin their treatment within 62 days.
Figures published on Thursday by the Department of Health for the final quarter of 2017 show the health service falling short of its treatment targets.
Ulster Unionist MLA and health spokesman Roy Beggs said it was "deeply disheartening" that cancer services had "suffered a further major deterioration".
"Compared to twelve months ago, the two key targets which cancer services are measured against have both worsened. It is outrageous that in December only 66.4% of patients started treatment for cancer within 62 days following an urgent referral - a further fall of over 2% on the year before, and despite the target being 95%. This is shocking and completely unacceptable," he said.
The other target referred to by Mr Beggs is for breast cancer services in Northern Ireland.
The government's target is for every urgent breast cancer referral to be seen within two weeks.
In December 2017, 1,022 of the 1,266 (80.7%) patients given urgent referral for suspected breast cancer were seen within two weeks - a drop of more than 10% from the 91.4% of patients seen within two weeks in December 2016.
Mr Beggs added: "This time last year I said that if the two larger parties could ever get their act together and form a functioning Executive, the next Health Minister urgently needed to focus their efforts on cancer prevention, early detection and rectifying the gaps within the local medical workforce which are causing the delays. That didn’t happen and now cancer waiting times have continued to get worse.
“The frightening thing is, as far as I can see, there is nothing being done to resolve the current crisis so I fear waiting times generally are doomed to get far worse before they get any better and the people of Northern Ireland will continue to suffer unnecessarily."