Bowel cancer patients are waiting too long for referral and treatment, although survival rates are improving, according to Audit Scotland.
A screening programme will be introduced next year
The public spending watchdog warned the Scottish Executive may not meet its waiting time target later this year.
Opposition parties said it was worrying that 40% of patients referred urgently were not treated within two months between July and September last year.
The health minister said the executive was determined to "drive up standards".
The condition has the second highest death toll of any cancer, with about 3,500 cases being diagnosed in Scotland every year.
The executive has invested more than £100m in modernising cancer services as a whole.
Audit Scotland's review of bowel cancer services found that survival rates have improved and the quality of care is high, with good co-ordination between teams in different parts of the country.
However, in the three months to the end of September only 60% of patients who had received an urgent referral began treatment within two months.
The report warned that the executive was unlikely to meet its target of a two-month maximum wait by the end of 2005 if current trends continued.
It also said that a shortage of qualified staff meant few of the endoscopy suites used to diagnose bowel cancer were used to full capacity.
Deputy Auditor General Caroline Gardener said: "Bowel cancer services in Scotland are performing well against clinical standards and waiting times are improving.
"However, there is still considerable progress to be made."
The report also warned that the introduction of national screening for bowel cancer in 2006 will put real pressure on services
Health Minister Andy Kerr said he was confident that this service, aimed at the over-50s, would make a difference.
"We recognise that resources need to be looked at as the screening goes nationwide and these will be taken on board as we roll out the screening across Scotland," he said.
Mr Kerr said the report provided "a positive endorsement" of the care provided to patients but also showed that there were challenges in areas like waiting times.
Tory health spokesman Dr Nanette Milne said the report provided "damning evidence" that patients were waiting too long.
She said: "While we welcome the progress made in the improvement of co-ordinating services and the role of specialist nurses, it is extremely worrying that only six in 10 patients in Scotland started treatment within the two-month target period and almost 70% of patients with bowel cancer were not given an urgent referral by their GP."
Scottish National Party health spokeswoman Shona Robison said: "This is a very serious failure in performance that is putting lives at risk.
"This must be a matter of utmost urgency for the health minister."