The Scottish Government set the ambition of having 90 per cent of patients beginning their treatment within 18 weeks of being referred, but new data showed performance has slumped to the lowest point since it was introduced in December 2011.
Other NHS figures showed a rise in people waiting for key diagnostic tests, while the number of patients waiting more than 12 weeks for inpatient or day case treatment has increased by almost 10,000 in two years.
The latest figures on patient referral to treatment time showed that in September, 81.4 per cent of patients started getting help within the 18-week target, with 16,528 waiting longer than this.
Figures for July and August showed 83.2 per cent and 82.4 per cent started treatment within 18 weeks.
The figures do not include patients in the NHS Tayside area, as health board bosses there were unable to provide data because of reporting problems following the introduction of a new patient management system.
Of the 14 health boards who provided valid figures, only three met the 90 per cent referral-to-treatment target in September. In NHS Grampian, two-thirds of patients began treatment within 18 weeks of being referred - the worst performance in Scotland that month.
Separate figures showed that at the end of September, there were 77,819 patients in the NHS waiting for a key diagnostic test - an increase of 19.2 per cent from the year before.
Almost one in five had been waiting more than the six-week target, with tests carried out within this time for 81.6 per cent of patients. That means the number of patients waiting more than this time almost doubled over the course of the year.
Conservative health spokesman Miles Briggs said: “With every week that passes, another set of statistics exposes the SNP’s shambolic running of our NHS. And while it may look bad on paper, it’s even worse for the patients and over-stretched medical staff who have to face the reality. It’s well known that the sooner someone starts treatment after being referred, the better their chances of recovery. Yet under this SNP government the referral-to-treatment waiting times have plummeted to their worst ever.”
Health Secretary Shona Robison said the Scottish Government is “committed to delivering the investment and reform necessary to ensure our NHS is fit for the changing needs of a 21st century Scotland”.