An elderly man died after waiting for more than seven hours in a hospital A&E department as a result of "dangerous overcrowding", the trust's medical director has said.
The 85-year-old man suffered a fatal heart attack as he waited to be treated.
He arrived at Northampton General Hospital NHS Trust's A&E department complaining of stomach pains and was initially seen within an hour, according to the Health Service Journal.
But he became unwell while waiting to be seen by senior staff after a blood test suggested heart problems, it said.
The trust's medical director, Matthew Metcalfe, said the pensioner's death was "entirely" caused by overcrowding in an email leaked to the HSJ.
The trust has apologised to the family of the man, but said it was not clear that he would have been saved if he was seen sooner.
It acknowledged that "ideally" he should have been dealt with earlier, and has pledged to carry out a full investigation "to ensure any learning from this sad incident is taken forward".
The pensioner, who has not been named, attended A&E just after 4pm on Wednesday with non-specific upper abdominal pain.
He was triaged at 5.30pm and blood tests revealed he was anaemic with a possible cardiac problem, so medics planned to give him a blood transfusion.
But his condition deteriorated just before 1am, and he suffered a cardiac arrest.
A spokeswoman for Northampton General Hospital NHS Trust said they were struggling with increased number of patients presenting at emergency departments.
"Ideally this patient would not have waited so long, would have been reviewed sooner by a senior consultant and might have been in a hospital bed on a ward at the time of his deterioration.
"We don't yet know what difference this would have made to the final outcome. The hospital's A&E unit has seen an average of 400 patients a day attending over the past few months, an increase of almost 30% on the same period last year.
"This has inevitably had an impact on our ability to consistently provide a high standard of patient safety in our ED."
The statement added that staff are "resolutely focused on patient safety".
"Although every effort is made to see, assess and treat patients quickly in our ED department and NGH has a good record in this regard.
"However, in this situation the long wait for further treatment and assessment led to an unacceptable outcome."
A spokesman said Mr Metcalfe's email had been sent to all consultants "to ensure they were fully aware of the seriousness of the position in our emergency department and also the importance of their ongoing support to our patients attending the department and their colleagues in the ED team."
The hospital today issued the following statement: