NHS Tayside has not hit a key target on treating sick patients since 2014, according to official figures.
The Scottish Government demands that health boards begin treatment on at least 90% of patients within 18 weeks of them being referred.
But Tayside has gone 15 months without achieving that, which has been blamed in part on bed-blocking.
Anas Sarwar, Scottish Labour’s health spokesman, said: “The SNP set our NHS targets and then don’t deliver the resources or management to achieve them.
“In Tayside the Referral to Treatment standard hasn’t been hit for nearly 18 months – a sure sign the sticking plater approach of the SNP isn’t working.
“This is SNP Health Minister Shona Robison’s backyard but we are seeing patients being let down, what’s more, hundreds of millions of pounds of cuts to public services will only increase the pressure on our health service.”
The last time Tayside hit 90% was back in December 2014 and their performance sunk to an all-time low of 84% in November last year.
It met the target for more than two-and-a-half years after it was introduced in March 2012, before the slump set in.
The figure is creeping back towards the target after Tayside posted 88% in the latest figures for March, which were released by NHS Scotland this week.
A spokeswoman for NHS Tayside said they are “committed to delivering high-quality, safe and effective care for all our patients”.
“The 18-week referral to treatment standard sets out that 90% of patients should have a completed pathway from referral to first treatment starting within 18 weeks,” she added.
“Since November 2015, NHS Tayside has improved its performance against the RTT standard despite the healthcare pressures that are experienced over the winter months.
“We are implementing a number of improvements across the care pathway to increase capacity which we expect to impact positively on our overall RTT performance.”
Ms Robison said Scottish health boards are delivering some of the lowest waiting times on record, but added more work is needed “to maintain and improve performance to meet the rightly demanding targets we have set”.
“Patients should expect nothing less. That’s why we have provided recent investment to ease pressure, and set out long terms plans to ensure our NHS is fit for the future,” she added.
That includes a £40 million new elective treatment centre for Dundee, one of five across the country designed to ease waiting times for planned surgery such as hips, knees and cataracts.
