It is understood that a report prepared for Home Office officials after the first week of a work-to-rule describes significant arrears in work which could considerably delay prospective nurses, teachers and social workers obtaining the necessary clearance to work with children and vulnerable adults.

The action has hit:
-Police volume management: where CRB staff provide 'enhanced' disclosures on behalf of police forces unable to cope with demand such as the Metropolitan Police, West Midlands and Avon and Somerset
-Production area: where the bulk of CRB work including 'standard' disclosures are produced
-Management checking unit: which provides quality assurance checking of work performed by the CRB private sector partners, Capita.

PCS members have been refusing to work beyond their targets or cover for absent colleagues since 13 October.

An overtime ban started on Saturday and is expected to compound matters for an agency which relies heavily on weekend working.

Negotiations officer Peter Middleman said: "We know that the CRB made a surplus of almost £10m last year and a tiny fraction of this could settle the dispute and prevent any further disruption.

"The work-to-rule, however, does demonstrate the extent to which the goodwill of staff has previously contributed to the CRB’s otherwise good performance and it's a shame that the employer refuses to recognise that."

The dispute centres around a 0.5% pay offer which was rejected by 91% of union members.

Meanwhile, the civil service is braced for yet more industrial action after 270,000 members voted on Friday for a series of strikes and industrial action over pay beginning in November.