Only 11.9% of urgent breast cancer referrals were seen within 14 days in the Northern Health and Social Care Trust during September, official figures have shown.
The target is that 100% of women should be seen within 14 days.
Where patients were not seen within 14 days, the longest wait was 44 days.
The Northern Trust said it had experienced a high level of demand for breast cancer awareness over the summer months.
A spokesperson said: "This resulted in an increased number of patients waiting greater than 14 days to be seen in the breast clinic.
"The trust took action to address this position by securing additional in-house breast assessment clinics in Antrim and also in a neighbouring Trust and as a result, October per improved to 57.5%."
It added: "From the 29th October 2018 the trust is back to seeing 100% breast red flag patients within 14 days of receiving the referral."
'A matter of urgency'
In August 2018, only 19.2% of urgent referrals in the Northern Trust area were seen within 14 days.
Where patients were not seen within 14 days during August, the longest wait was 25 days.
In a statement the Health and Social Care Board said: "We acknowledge the Northern Trust performance in recent months has deteriorated significantly due to increased demand for breast assessment services and a short term staffing issue.
"This has now been resolved and the October position against the 14 day standard is at 57.5% and we expect this improvement to continue."
"For some time, the trust has been undertaking additional activity to meet patient demand however, due to bank holidays and staff annual leave in July, capacity has been insufficient to meet demand and a backlog of patients has accumulated as a result."
Ulster Unionist Health spokesperson Roy Beggs MLA said he had been aware of the situation regarding breast cancer referrals in the Northern Trust area for some months but had chosen not to highlight it in order to avoid unduly raising patient concerns.
"But the current situation now has become so serious, and the figures so bleak, that unless immediate action is taken I really am fearful that patients will come to harm," he said.
"Targets are set for cancer treatment because there is very sound medical evidence that the longer a patient has to wait for treatment, the greater the risk that they may ultimately come to harm.
"Cancer is an especially cruel and vicious disease that thrives during any avoidable delay in treatment. It is also totally unfair to force patients who think they may have cancer to wait so long for answers."
In relation to other trusts, 100% of urgent referrals were seen within 14 days during September in Belfast, South Eastern, Southern and Western Trusts.
An increase in waiting times in a number of trusts including the Southern Health Trust has prompted a review of breast cancer assessment services in Northern Ireland.
This is when a woman is referred for further tests after an initial breast cancer screening at her local health trust.
BBC News NI understands that the plan is to reduce the current number of specialist breast cancer services and instead provide so called centres of excellence.