Obesity surgery is often seen as a quick fix, without proper consideration of the risks, a review says.
The National Confidential Enquiry into Patient Outcome and Death looked at the care given to more than 300 patients at NHS and private hospitals in England, Wales and Northern Ireland.
It found that many were given insufficient time or information to properly consent to the operations.
Post-surgery care was also found to be lacking, the watchdog said.
In particular, it highlighted the fact patients were not always given access to dieticians and psychologists.
The report also suggested the failings could be contributing to the high number of readmissions - nearly a fifth of the patients had to return within six months.
Weight loss operations, such as the fitting of gastric bands, have been growing in popularity.
There were more than 8,000 of these operations, sometimes called bariatric surgery, carried out by the NHS last year - and the number is rising by about 10% a year.
The numbers paying for the surgery themselves is unknown.
The review found only 29% of patients received psychological counselling prior to referral for surgery and in a quarter of cases consent forms did not contain the appropriate information about the surgery, including the risks involved.
Follow-up care was deemed to be inadequate in nearly a third of cases.
The report also took issue with the way the surgery was being portrayed in advertising, saying many adverts only showed it in a positive light.
The watchdog has asked professional associations and regulators to draw up a code of conduct.
Report co-author Ian Martin said: "Bariatric surgery is a radical procedure with considerable risks as well as benefits.
"It shouldn't be undertaken without providing full information and support to patients.
"Consent often happens on the day the patient is admitted for surgery.
"This means there is no time for patients to reflect on their choices and have the opportunity to ask further questions about the risks and benefits of surgery before committing themselves to an operation."
But Royal College of Surgeons president Prof Norman Williams said work was already under way to ensure standards were being maintained.
He said new clinical guidelines were issued earlier this year, covering many of the issues raised.
He also said that research had shown that bariatric surgery was proving to be an "incredibly successful and cost-effective treatment".
"We will continue to work together with other health professionals in this area of surgery to ensure high standards of care and patient safety are maintained," he added.