Patients could have been put at risk of serious harm after NHS services in England were outsourced, a report says.
Nearly 90 women were wrongly told they were no longer part of the cervical screening programme after Capita started running back-office services in 2015, the National Audit Office said.
It said services had been "way below" acceptable standards, although no harm to patients had been found.
NHS England acknowledged "difficulties" but said the change had saved £60m.
NHS England agreed a seven-year £330m deal with Capita in 2015 to run back-office functions for primary care providers such as GPs, dentists and pharmacists in a bid to reduce costs and modernise services, the public spending watchdog said.
Duties transferred to Capita included sending out test results, moving patients' medical records, processing patient registrations and paying GP practices.
The report found patients could potentially have been put at risk because of problems with the "performers list" - a list of GPs, dentists and opticians practising in the NHS - including whether they were suitably qualified and had passed other relevant checks.
"The failure to update performers lists may have compromised patient safety in cases where practitioners should have been removed," the authors of the report said.
Other issues reported by the NAO included:
- Delays in processing new applications for the performers list in 2016 resulted in about 1,000 GPs, dentists and opticians being unable to work
- Capita carried out an "aggressive" office closure programme that it continued despite the fact it was having a harmful impact on services
However, the NAO acknowledged that NHS England had made savings of £60m in the first two years of the contract, and said Capita's self-reported performance against the contract had improved.
But Sir Amyas Morse, the head of the watchdog, added that value for money was "about more than just cost reduction".
"It is deeply unsatisfactory that, two and a half years into the contract, NHS England and Capita have not yet reached the level of partnership working required to make a contract like this work effectively."
The NAO added that NHS England should consider whether the services should be taken back in-house.
Meg Hillier, chairwoman of the Commons Public Accounts Committee, said trying to cut costs while modernising the service was "over-ambitious, disruptive for thousands of doctors, dentists, opticians and pharmacists and potentially put patients at risk of serious harm".
Dr Richard Vautrey, chairman of the British Medical Association's GP committee, said it was asking NHS England how it planned to resolve the "shambles" of Capita running the primary care support services.
'Too many unknowns'
An NHS England spokesman said the transition, "while not without its difficulties", had saved £60m which had been reinvested into front-line care, funding the equivalent of an extra 30,000 operations.
A Capita spokeswoman added: "The report notes that several organisations and legacy issues all contributed to underperformance.
"It has been acknowledged that performance has improved and Capita will continue to work with all parties to address the small number of remaining service issues."
Separately, earlier this month Health Secretary Jeremy Hunt said up to 270 women in England may have died because they did not receive invitations to a final routine breast cancer screening.
He said IT problems were to blame for the errors.