The increase in the NHS carrying out operations as day surgery has saved around £2 billion, according to the chief economist of a leading think tank.
John Appleby, of the King's Fund, said a "revolution" in surgery over the past 40 years has led to a large-scale switch to day cases, which cost considerably less than keeping patients in overnight.
Writing in the BMJ, he said figures show that in England the proportion of all elective and non-elective procedures that were carried out as day cases each year went from a low of around 7% in 1974 to nearly 35% by 2013 - an increase from around 417,000 to 6.3 million cases a year.
He wrote that, while in 2013/14 the average cost for a day patient was £698, the average elective inpatient cost was £3,375.
"Imagine if the switch to day cases had not changed," he said.
"Based on national reference costs reported by English hospitals, the total cost of treating the total 6.96 million elective day and inpatients in 2013 was around £8.9 billion.
"To treat this number of patients, but with the proportion of day cases as observed in 1998, would cost nearly £11 billion (over 22% more). By treating more patients as day cases, the NHS had in effect saved around £2 billion by 2013."
He said this was likely to be an overestimate, as those who changed from inpatient care to day cases are likely to have had less complicated conditions and would therefore have had shorter stays and been less costly to treat anyway.
Dr Appleby added that although day cases now account for nearly 80% of all elective procedures each year, there is still scope for further increases, with current variations across England for surgeries such as haemorrhoidectomy and bladder tumours.
He said that, assuming the proportion of day cases continues to increase at the same rate for the next decade as it has done in the 15 years since 1998, then the total spent on elective care in 2013 would pay for 22% more patient episodes in 2023.
The article is based on a recent report by the King's Fund, Better Value In The NHS.