More than 13,000 hospital operations in Wales have been cancelled over three years due to a lack of beds and staff, figures from health boards show.
Cancellations due to bed shortages have risen 72% since 2010/11.
The Welsh government said hospitals faced spikes in pressure during the winter, and that a "high proportion" of operations were cancelled by patients themselves.
But the Conservative opposition blamed a lack of funding for the NHS.
The figures were obtained through Freedom of Information (FoI) requests by the Western Mail newspaper.
The number of cancelled operations in each health board due to bed or staff shortages from to 2010/11 to 2012/13 was:
- Abertawe Bro Morgannwg - 1,755
- Aneurin Bevan - 3,192
- Betsi Cadwaladr - 1,135
- Cardiff and Vale - 4,442
- Cwm Taf - 1,356
- Hywel Dda - 1,296
- Powys - None
Conservatives in the assembly accused the Welsh government of cutting funding in real terms for the health service by failing to ensure it rises in line with inflation.
Shadow health minister Darren Millar said: "These shocking figures reveal the scale of operation cancelling in the Welsh NHS.
"Cancelling long awaited operations not only wastes millions of pounds every year but causes enormous distress to vulnerable patients and can negatively impact on their long-term treatment and prognosis."
He added: "Labour Ministers must listen to those who work in the NHS and who need the resources to deliver the standards of care which patients expect and deserve."
A Welsh government spokesman said: "We are committed to reducing the number of cancelled operations across Wales.
"There will always be occasions when operations have to be cancelled due to clinical reasons or patient choice.
"However during the winter months the NHS in Wales faced unprecedented unscheduled care pressures which affected planned care activity.
"A large amount of operations are carried out across Wales every year. Of these a high-proportion are cancelled by patients themselves.
"Sometimes it will be necessary for a hospital to cancel an operation for justifiable and professional clinical reasons."
The Welsh government said that patient cancellations accounted for about 40% of total cancellations in recent years, and that total cancellations due to bed or staff shortages over three years was equivalent to around 1.6% of the total number of operations carried out in that time, around 800,000.