Its investigation of the Business Appointment Rules also discovered the rules do not say that a department can reject an application, only that these may be accepted or have conditions attached.
The rules apply when civil servants move to outside bodies and are designed to prevent them using privileged information or contacts gained in the civil service or to prevent any perception of impropriety.
They are administered by the Cabinet Office but the NAO found it removed guidelines for departments on administering the rules in 2012 to write fresh ones, which have never been completed.
The lack of clarity over rejection of applications even saw the Cabinet Office tell the NAO it thought such rejections did in practice happen, but was unable to supply any evidence.
Nor did the Cabinet Office maintain oversight of how departments implemented the rules, relying on them for enforcing compliance, transparency and public scrutiny.
Auditors also found it impossible to discover from transparency data whether all those leaving the civil service for the private sector who should have made an application under the rules in fact did so.
Departments have since 2014 been required to publish the outcomes of business appointment applications but only 170 had been issued and three unnamed departments had have never published while one made a nil return.
The NAO was unable to find any department that had assurance that former civil servants followed the requirement to remain compliant with the rules for up to two years after they left public service.
The Advisory Committee on Business Appointments (Acoba) makes indepedent judgements on the most senior civil servants and ministers. Its role did not fall within the scope of this NAO report. However, Acoba was criticised by the Commons Public Administration and Constitutional Affairs Committee in an April report, which concluded that was “ineffectual and does not inspire public confidence or respect”
Transparency International UK policy director Duncan Hames said: "The rules in the Civil Service Code only give the impression of regulation of public officials to protect against the risk that the ‘revolving door’ works against the public interest.
“Instead, what we have is an inconsistent system of polite requests and potential discouragement, with no evidence of appointments ever being rejected, no monitoring of compliance with conditions, and therefore no enforcement when they are breached.”
Hames said Acoba provided only “a mirage of oversight” and called for its replacement by a body with statutory powers to monitor compliance and apply sanctions.
A Cabinet Office spokesperson said: “As the NAO has acknowledged, this is an issue that government takes extremely seriously.
“The importance of complying fully with the business appointment rules is set out in a civil servant’s terms and conditions of employment, and these are therefore legally binding. We will carefully consider the full report and its findings.”
