An investigation into safeguards intended to stop senior civil servants exploiting their Whitehall knowledge when they switch to the private sector has found that Government departments do not know whether conditions placed on ex-employees are being complied with.
The National Audit Office (NAO) probe also said that it was not clear that all mandarins who should have gone through the checking process, known as the Business Appointment Rules, had done so before moving to outside posts.
The rules are meant to prevent top civil servants from taking sensitive government information, contacts, or knowledge to their new employer, but the report said the system is not being applied consistently across Whitehall.
The investigation by the Comptroller and Auditor General into eight Government departments found most believed "the onus to be on former civil servants" to comply with any conditions placed on them, such as not to lobby former colleagues.
The report noted that a parliamentary watchdog had said expecting journalists to expose breaches of the rules was wrong.
"The Cabinet Office relies on transparency, public scrutiny and reputational risk to promote compliance with the rules. The Public Administration and Constitutional Affairs Committee considers that it is completely unacceptable to rely on media coverage to expose perceived or actual breaches of the rules, and 'trial by media' only serves to further weaken public confidence."
The study said: "No department has assurance that former civil servants remained compliant with the rules for up to two years after they had left public service.
"It is not possible to know from transparency data whether all those leaving the civil service that should have made an application under the rules did so."
The report found most departments do not inform outside employers of conditions imposed on departing civil servants.
"Departments must inform prospective employers of any conditions attached to the approval of an appointment or employment. In our sample, only one department consistently informed prospective employers of conditions attached to a business appointment approval, as required by the rules.
"The remaining seven departments have attached conditions to at least 187 approvals in the past five years. However, only two departments have informed prospective employers of conditions attached to an approval, and in total only six notifications have been sent.
"One department told us that it never writes to prospective employers, but it expects leavers to share the department's decision with their new employer."
Despite a 2014 decree to publish outcomes of business appointment applications by civil servants, three departments had never done so, the probe said.
The report found that guidelines for administering the rules have been removed from the Cabinet Office's civil service management code, and a replacement document has been under preparation since 2012 but not yet published.
The probe said the rules do not state that departments can reject applications, and when the Cabinet Office said it believed some had been rejected it could not provide any evidence of this.
The Cabinet Office does not exercise its right to monitor how the rules are being applied across Government, meaning there is no central oversight, the inquiry said.
Jon Trickett, shadow minister for the Cabinet Office, said: "It is often said that there is a 'revolving door' where members of the elite only take care of themselves. With the Tories, the revolving door spins even faster.
"This National Audit Office report shows how little the Government cares about members of their cosy club passing from Whitehall to big business. There is complete lack of oversight and the system is toothless.
"The Tories have even weakened the rules so that they don't apply to many special advisers - meaning spinners can go straight into lobbying using their connections and privileged information. It's a scandal waiting to happen.
"The British public have cottoned on to how money and power stays within a closed circle."