Tens of thousands of operations were cancelled by English hospitals last year but were never included in official figures.
A quirk in how hospital data is recorded means that operations cancelled on the day of admission or the day of the planned surgery are counted, but those in the few days beforehand are not.
The BBC used the Freedom of Information Act to obtain data from all 156 NHS trusts in England.
They provided figures for operations cancelled one to three days before a patient was due to be admitted.
NHS England figures showed nearly 75,000 NHS operations were postponed at the last minute in 2015/16 – a 10-year high
Of 74 that gave data, 41,474 operations had been cancelled within that period – compared with 33,400 in the official figures.
The main reasons were a lack of beds or ward space and staff shortages.
Official NHS England figures show about 7.7 million planned operations were carried out in England last year.
There were 71,370 last-minute cancellations, plus thousands more that did not get recorded.
The main reasons for the operations being cancelled at the last minute were a lack of beds or ward space and staff shortages
An NHS England spokeswoman told the BBC: ‘The proportion of patients seeing their operations cancelled at the last minute remains under 1 per cent in spite of record numbers of operations being scheduled.
‘Our national data collection rightly requires trusts to focus on monitoring the number of last-minute cancellations, as this is where the most distress is caused for patients.
‘Hospitals should continue to ensure that every effort is made to reschedule cancelled operations as soon as possible.’
Katherine Murphy, chief executive of the Patients Association, told the BBC: ‘You can’t underestimate the upset for them, their family, their friends and everybody else if an operation is cancelled.’
She said the association heard about the impact of cancelled operations ‘every day’.
She added: ‘We are talking about individuals who are often in pain or suffering. They will have made lots of arrangements.
‘It can have a huge psychological or financial impact on people.’