Half of the 251 security firms audited this year by the police received grades of C or D, according to a statement by the police on Tuesday. This was the industry's worst result in recent years.

The annual audit scrutinises a firm's operations, management and manpower quality, among other criteria. The grading is based on the relevant documents submitted by the security agencies as well as on-site checks.

The firms are given grades of A to D, with C being satisfactory and D unsatisfactory. About the same number of firms were graded in 2012, but the percentage of firms getting the D grade shot up this year from 13 per cent to 21 per cent.

The police said in a statement that "security officers are now better equipped and trained, and have better employment terms and job prospects. Security agencies should constantly improve their employment and work place practices so as to attract and retain a skilled workforce".