The number of people waiting to be admitted to hospital from A&E departments in England has hit record levels, as new figures show the scale of pressure the NHS has faced this winter.
Almost 80,000 patients were kept waiting more than four hours, while nearly 1,000 faced a wait of over 12 hours.
Health charity the Nuffield Trust called the figures "dismal reading" and said corridors had seemingly become "the new emergency wards".
Another key measure of NHS performance, the proportion of people seen within four hours of arriving at A&E, was the lowest on record: 85.1%. The target is 95%.
Some 1.9 million attendances were recorded at A&Es in January, down slightly on January 2016, but total attendances in the 12 months to January showed a year-on-year increase of 4%.
Jim Mackey, chief executive of NHS Improvement said: "January was an exceptionally busy month for our A&E departments and these figures show the impact that the continued high patient demand had.
"The pressures the NHS has been facing have been intense but, despite this, we have to recognise that NHS staff managed to see almost nine out of 10 patients within the four-hour target time.
He added: "The extra £100m announced in the Budget will help the NHS take steps in A&Es to help make sure more patients get seen quickly and in the right place for their needs, but there is plenty more that we need to do across the system."
The figures, from NHS England, also show the number of people waiting more than two months to start cancer treatment after an urgent referral hit a new high of 2,437, or roughly one in five of patients seen by GPs.
The target for this type of referral is 85%, but in January the level was just 79.7%, the lowest on record.
Emma Greenwood, Cancer Research UK's director of policy, described the numbers as "completely unacceptable".
"Cancer targets exist to ensure quick diagnosis and access to treatment, and provide a snapshot of how the NHS is performing for patients," she said.
"The Government and NHS England have committed to improving early diagnosis of cancer, including increasing investment, but it's clear that this is yet to have an impact.
"Urgent action is needed now to give our patients the care they deserve and the best chance of surviving."
January's figure for delayed transfers of care, so-called "bed-blocking", were the second-highest on record.
The total number of hospital days lost through delayed transfers were 197,054, up 23% on January 2016.
Nuffield Trust chief economist and director of research John Appleby said: "The numbers of patients stuck on a trolley waiting for a hospital bed have gone through the roof.
"These are vulnerable people with acute medical needs.
"The problems in social care are well known and cuts to services have been a big driver of these problems.
"That's why the extra money announced in yesterday's Budget is welcome.
"But with the NHS experiencing its own pressures and the social care funding gap set to be at least £2 billion in the coming year alone, there are no guarantees that patients at A&E can expect let-up any time soon."
Health Secretary Jeremy Hunt said: "NHS staff have worked exceptionally hard this winter and, despite the additional pressure, treated more than 1.6 million people in A&E within four hours in January.
"We have however seen a number of unacceptable cases of poor performance and patient care that cannot be repeated.
"That's why NHS England has today launched a plan to get hospitals back on track to meet the 95% standard, supported by the Budget announcements of £100 million for A&E and the £2 billion social care funding that will help to improve hospital discharge."
The plan was outlined in a letter from NHS England chief executive Simon Stevens, which has been sent to all NHS trust chief executives and primary care leaders.
In the letter, Mr Stevens said there was a need to "make concrete changes" to ensure A&E departments do not suffer next winter in the way they have this one.
Among the measures listed was ensuring GPs open for longer hours, and having more callers to the 111 helpline dealt with over the phone to ease pressure on A&E departments.