The NHS in England has missed its four-hour A&E waiting time for the first time this winter, figures show.
In the last week, 94.8% of patients were seen within four hours compared to 95.6% the week before. The target is 95%.
The figures cover all centres - major A&Es, smaller minor injury units and urgent care centres.
Waiting times are worst in the major units where only 92.2% were seen within four hours.
The figures show two thirds of the 144 trusts with major units are missing the target.
It is the first time since April the target has been missed overall - although the major units have been below the 95% mark since July.
It is not unusual for performance to drop in December.
For the last three years individual weekly figures have fallen below 95% before Christmas.
Last winter the NHS was consistently below the 95% figure overall from January to April.
Hospitals are given a 5% leeway to allow doctors to prioritise the sickest patients.
NHS England chief operating officer Dame Barbara Hakin said it was "disappointing" performance had dropped, but said last week was the busiest so far this winter with more than 415,000 people attending A&E, while the numbers that needing admitting to hospital for further treatment - emergency admissions - hit its highest level since 2010.
She added: "We know the A&E standard is ambitious and that is only right. This is the first week since April the 95% standard has not been met, however we do know that sometimes this will happen.
"Every year we see a dip in the figures for December, with week on week variations which is why we fully assess how local systems are coping with winter pressures over a longer period.
"We knew this winter would be difficult but it is important to stress the NHS continues to deliver a good service. This is thanks to the hard work and dedication of our frontline staff."
The drop in performance comes despite the government giving the NHS extra money to cope with winter - £250m was announced in the summer and another £150m was pledged last month.
A Department of Health spokeswoman added: "We have always been clear that this could be a difficult winter - and there could be more difficult weeks ahead. But the majority of patients continue to get the excellent care they deserve."