The percentage of people seen within four hours of arriving at A&E is at its lowest level since April, according to new figures.
Data for England shows 94.3% of patients were seen within four hours in the week ending January 5 - against a national target of 95%.
In major A&E departments, the figure was also at its lowest level since April, with 91.5% of patients seen within four hours.
The figures come despite a drop in attendances, with visits down compared to a normal winter week.
Around two thirds of the 144 trusts that run major A&E departments are now missing the target.
It follows a tough week for A&E.
Earlier this week, the health regulator said patients at one of the busiest A&E departments in the country had been put at risk because of staff shortages.
The Care Quality Commission (CQC) said patients' safety and health and welfare needs were not being met at Bradford Teaching Hospitals NHS Foundation Trust.
Meanwhile in Belfast, a major incident was declared at Belfast's Royal Victoria Hospital due to a large backlog of patients at the A&E department. Some faced long waits on trolleys.
The four hour target is regularly missed by hospitals during the winter months. It cover all centres - major A&Es, smaller minor injury units and NHS urgent care centres.
Today's data also showed the number of delays in ambulances being able to hand over patients to A&E was 5,234 between January 2 and 8.
Over Christmas week, from December 24 to January 1, the figure was 4,937.
These are delays of more than 30 minutes and come after Dame Barbara Hakin, chief operating officer of NHS England, admitted to MPs in December that the NHS has a "significant problem" with ambulances queuing at A&E departments.
She was answering accusations that ambulances are "stacked outside A&E departments like planes".
Dame Barbara said: "Despite the traditional pressures on health services in the weeks after Christmas, the NHS continues to deliver for patients.
"We have seen 94.3% of those attending A&E treated, admitted or discharged within four hours during the week ending January 5.
"We are really pleased that the A&E target, measured quarterly, has been met for quarter three.
"The first few weeks of the calendar year are generally the toughest for the NHS, but this week's figures, the first week in quarter four, are over a percentage point better than the same week last year. This is hugely encouraging for patients and follows an excellent performance over the Christmas holiday period.
"All this suggests that the NHS has prepared well and pulled out all the stops on behalf of the public."
Dame Barbara said there were 2,674 more emergency admissions than the same week last year, though overall A&E attendances are falling.
She added: "Encouragingly, the number of A&E diverts and ambulance handover delays were down significantly compared to the same period last year. The number of cancelled operations were also down. The number of beds closed due to diarrhoea and vomiting or norovirus-like symptoms remain stable.
"However, the number of delayed transfers of care were up by 278 on the same week last year. We have asked local health leaders to redouble their efforts on this issue so as to spot issues early and take action.
"For the first time this winter, there have been two reported closures. However, these were services that receive patients with minor injuries only. In this instance, Wye Valley NHS Trust took an operational decision to redeploy nursing staff from two minor injury units to support their major emergency department on a temporary basis."
A Department of Health spokesman said: "The NHS has missed its A&E target in the first week of January every year for the past decade.
"We know winter can be tough. That's why we've taken early action to help the NHS cope, investing £400 million in alleviating short term pressures and A&E performance is better than this time last year.
"Longer term, we're bringing back the link between GPs and elderly patients and investing £3.8bn in joining up health and social care.
"The vast majority of patients continue to get the excellent care they deserve. That's because of the hard work of staff and we want to thank and support them in that."
Shadow health secretary Andy Burnham said: "Today's figures show a worrying slump in A&E performance that should set alarm bells ringing in 10 Downing Street.
"Experts warned the Government about the A&E crisis but they refused to listen. Just weeks after ministers said 'the crisis is behind us', patients have experienced the worst week in A&E so far this winter.
"David Cameron's fingerprints are all over this crisis. He wasted billions on an unnecessary reorganisation and threw the NHS into chaos. He's made it harder to get a GP appointment, closed a quarter of NHS walk-in centres and made deep cuts to social care support - all the ingredients for the current pressures on A&E.
"The A&E crisis is yet more proof that the NHS is not safe in David Cameron's hands."