Calderdale and Huddersfield NHS trust has lost almost £1.5 million this year due to thousands of patients not turning up to appointments, figures show.
Data from NHS England shows that, between January and June, 11,931 people either did not show up for an outpatient appointment at the trust, or arrived too late to be seen.
With the NHS struggling for funds amid budget cuts and increased demand, the British Medical Association said it was crucial appointments are not wasted while the health service is "under incredible stress".
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The average outpatient appointment costs the NHS £120, according to the latest resources cost data.
This means that the 11,931 missed sessions cost Calderdale and Huddersfield NHS Foundation Trust around £1.43 million.
Dr Robert Harwood, chairman of the BMA's consultant committee, said: “It is important that no appointments are wasted at a time when the NHS is under incredible stress.
"We should not stigmatise patients who may for legitimate reasons be unable to attend.
"However, we do need the NHS to emphasise through clear publicity to the public that given the current unprecedented pressure, patients should make every possible effort to rearrange their appointment so that another person is able to receive treatment in their place.”
At Calderdale and Huddersfield NHS Foundation Trust, out of the 169,790 outpatient appointments, 7 per cent of patients did not show up.
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The figures show 4,435 people failed to make their first appointment, 7 per cent of first attendances, while 7,496, or 6 per cent, did not appear for a subsequent meeting.
Niall Dickson, chief executive of the NHS Confederation, which represents organisations across the healthcare sector, said: “The NHS is short of funding, short of staff and faces ever rising demand for its services.
"With modern communication, the excuses for missed appointments are running out. There will always be some unforeseen circumstances but in most circumstances, it should be possible to cancel appointments.
“Our members across the NHS are doing their bit – many hospitals and other services send out email and text reminders, and increasingly patients can check, book and cancel appointments on line.
"We would all acknowledge that the NHS can do more and using technology better will make life easier both for patients and the service. But patients can also do their bit – making the NHS as efficient as it can be, is in everyone’s interest."
Across health providers over England almost 2.9 million appointments were missed between January and June, which cost the NHS around £350 million.
Patients who used London North West University Healthcare trust were the worst at showing up to appointments, while in Cambridge people were the most reliable.