An elderly man died while waiting in an accident and emergency unit because of the "dangerous overcrowding of the department", the trust's medical director has said.
An email leaked from Northampton General Hospital NHS Trust to the Health Service Journal (HSJ) said the patient died after going to A&E with stomach pain on Wednesday afternoon.
The 85-year-old was initially seen and assessed within an hour, and was waiting on a chair to be seen by senior staff after a blood test suggested he may have heart problems, the HSJ said.
But he deteriorated and suffered a cardiac arrest.
The email from the trust's medical director, Matthew Metcalfe, said: "Last night a patient died due entirely to the dangerous overcrowding of the department.
"The risk we have all been aware of, but may have felt hypothetical, has just happened."
A spokeswoman for Northampton General Hospital NHS Trust said the email was 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".
"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," the trust said in a statement.
"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.
"Staff at NGH are resolutely focused on patient safety and are proud of their efforts in this regard.
"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."
The trust has apologised to the family of the man, who 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.
The trust said it would carry out a full investigation into the death "to ensure any learning from this sad incident is taken forward".