Maternity services at Derriford Hospital in Plymouth have been judged the worst in Devon and Cornwall.
The hospital was marked bad for poor training of staff
Derriford blamed the result on failing to supply all its statistics to the Healthcare Commission on time.
The hospital, which was given the lowest one-star ranking, said it would have been a grade higher, with two stars, if it had complied.
But that would still have left it below all other NHS trusts in the region, most of which got four stars.
Northern Devon, Royal Cornwall and South Devon NHS trusts received the maximum four stars.
The Royal Devon and Exeter Hospital received three stars.
The Healthcare Commission said Derriford's maternity unit was marked as bad for the inappropriate use of Caesarean sections and poor for the training of its staff in maternity skills.
Sue Williams, the head of midwifery and women's services at Derriford, insisted the department was "performing well", and in future the hospital expected a "far better" rating.
"However we recognise that we have failed to provide all the necessary information required for this assessment exercise and acknowledge that on this occasion we have let ourselves down."
The review of maternity services in England was launched in response to concerns from patients and staff.
The Royal College of Midwives says 5,000 extra midwives are needed, to cope with the rising birth rates.
However, the maternity service at Derriford is fully staffed with 195 midwives.
John Richards, chief executive of Plymouth Teaching Primary Care Trust, which commissions maternity services in Plymouth, said: "The issues identified by the Healthcare Commission, which arise largely from errors in reporting, do not give rise to any significant concerns about the safety or quality of the services provided by Derriford Hospital."