Oakhill Secure Training Centre failed to meet minimum standards for staffing levels last year, with no social worker employed at the centre for six weeks last summer.
The information was included in an answer to a parliamentary question, tabled by the Liberal Democrat peer Baroness Stern.
A Youth Justice Board spokesperson said a social worker from Milton Keynes Council worked at the centre "as a part of a link team" during the period, from 28 July to 11 September.
An interim social worker has now been appointed.
Last June the Commission for Social Care Inspection found arrangements for safeguarding children and young people were "effectively reliant" on its head of residence, who was the only qualified social worker on the staff team.
An unannounced inspection in November 2005 called for staff to be trained in child protection and investigations of child protection concerns. But it said due to changes in the senior management team this requirement had not been achieved when last year's inspection was carried out. CSCI carried out an unannounced inspection of Oakhill last month.