Some 124,000 patients had to wait in accident and emergency departments in England for at least 12 hours last year, according to an investigation.
The number has nearly doubled in two years, fuelling concerns the NHS is in crisis.
And doctors warned that "crowding kills" as long waits in casualty departments are linked to higher mortality rates.
Data from the Health and Social Care Information Centre, seen by BBC Radio 5 live Investigates, shows the figure has surged from the 68,000 incidents of patients waiting at least 12 hours in 2013.
Dr Adrian Boyle, chairman of the Quality Emergency Care Committee at the Royal College of Emergency Medicine (RCEM), told the programme: "It's not just about tedious waits for patients.
"Studies have repeatedly shown that there is an increased mortality in people who spend a long time in emergency departments, so crowding kills patients."
The data includes all patients still in A&E after 12 hours but does not give the reason for such long stays, so it will include those waiting to see a doctor, waiting for a bed on a ward to be found for them, and those who waiting to be discharged.
Dr Boyle said: "These are almost always people who need admission to hospital, so these are the most vulnerable and the most seriously ill patients."
Unlike the rest of the UK, the NHS in England does not publish data on patients who spend more than 12 hours in A&E from the moment they arrive in casualty.
Instead it starts the clock running only once a doctor decides a patient should be admitted, not taking into account the time they have already spent there.
As a result, trusts in England reported just 1,282 cases where the patient had to wait more than 12 hours in the year ending November 2015.
Dr Boyle called for NHS England to report 12 hour breaches from the moment a patient arrives in A&E.
The current NHS target is for 95% of patients to be discharged, transferred or admitted into hospital for treatment within four hours.
NHS England said: "The focus in England is the four hour standard to admit, transfer or discharge, and this applies to all attendances at A&E. For those who require admission we monitor the length of wait from decision to admit to admission. These figures collected by NHS England are published monthly."
:: BBC Radio 5 live Investigates is broadcast on BBC Radio 5 live on Sunday February 7 at 11am.