The NHS in England has missed a key cancer target for the whole of 2014, official figures show.
At least 85% of patients should start treatment within 62 days of being referred by GPs.
Between October and December, just 83.8% were treated within time, and this was the fourth successive quarter that the target was missed.
NHS officials say they are scrutinising the figures to unearth any problems.
Early diagnosis and treatment are vital for improving the odds of recovering from cancer.
The NHS has a series of targets to ensure this happens.
Some are being met, including the "two-week wait target" for getting 93% of patients to see a specialist within a fortnight of a GP referral.
And the latest figures show some 44,000 additional patients were seen within this time compared to the same quarter last year.
But in terms of treatment, for the past four quarters respectively from January to December 2014, just 83.8%, 83.5%, 84.1% and 84.2% of patients had their first therapy within 62 days of being referred by GPs.
Breakdowns for individual cancers in the previous quarter (October-December 2014) show the proportion of patients treated on time:
- breast cancer - 96.5%
- lung cancer - 76.2%
- lower gastrointestinal cancer - 73.8%
- urological cancers - 79.2%
- skin cancers - 94.6%
Sean Duffy, NHS England's national clinical director for cancer, said: "Swift diagnosis of cancer is key. Latest figures show that, compared to the same quarter last year, over 44,000 more people saw a specialist within two weeks of visiting their GP, which is good news for catching cancer early.
"But it's crucial we focus on maintaining waiting time standards for treatment as demand increases, so we are closely scrutinising these figures to pinpoint any issues on the ground.
"We have also created an independent taskforce to develop a plan to improve prevention, diagnosis and treatment over the next five years, with the aim of saving thousands more lives."
Sarah Woolnough, at the charity Cancer Research UK, said: "It's encouraging to see a slight improvement - something we hope will continue - but the '62 day target' for cancer treatment hasn't been met for a full 12 months.
"This isn't just about missed targets - consecutive breaches mean thousands of patients are being failed.
"These targets exist to ensure swift diagnosis of cancer and access to treatment, which is vital if we're serious about having the best survival rates in the world."
Andy Burnham MP, Labour's shadow health secretary, said: "In the last year, thousands of cancer patients have waited too long for treatment to start and, in some cases, their chances of survival have been harmed.
"When it comes to cancer, speed is everything. Labour is committed to cancer tests and results within one week to help end this scandal."
The latest figures from Wales show that 88% of patients are receiving treatment within 62 days - a higher proportion than England's.
But it is still short of the 95% target the Welsh government sets.