The Welsh surgeries linked to eHarley Street are Brynmawr Medical Practice; Blaenavon Medical Practice; Pontypool Medical Centre; Bryntirion Surgery, Bargoed; Tredegar Health Centre; Aberbeeg Medical Practice; Gelligaer Surgery, Hengoed; The Corporation Road Surgery, Cardiff, and Lliswerry Medical Centre, Newport.
BBC Wales understands at least 37 clinicians are affected.
Dr Ian Jones, who also worked at the Brynmawr practice, said he was owed more than £2,000, and BBC Wales has seen correspondence between him and eHarley Street about the money.
A further doctor, Dr Hussein – who wanted only his surname used – worked at Lliswerry Medical Centre in Newport, and said he was owed £20,000 for locum work.
Dr Hussein, a doctor for 25 years, was concerned about the service being offered to patients.
“I feel very, very, very sorry to my patients,” he said.
“They’re not giving the appointments and services needed,” he added.
Dr Hussein said he understood eHarley Street was running the surgery on a day-to-day basis.
