Fewer private security firms failed an annual police audit this year.

About 9 percent of 242 security agencies audited received the lowest D grade, down from 21 percent of 251 firms last year.

There was relatively little change in the number of firms at the top end of the league - 41 firms got the highest A grade this year, from 40 last year.

Getting a D grade indicates a firm's performance was unsatisfactory. The bulk of the security firms - 39 percent - were graded C, or satisfactory.

Firms were audited in areas such as how they organise their security operations, how their security officers are trained, and their human resource practices.

The police started the compulsory yearly audits in 2009 to professionalise the security agencies sector. New firms are exempted from the audit.

Security agencies said that the toughest area of audit was human resource practices.

"Areas like salary payment and welfare were scrutinised," said Mr Baljit Singh, managing director of Globalmax Security and Consultants, which received an A grade.

Mr Robert Weiner, president of the Association of Certified Security Agencies, which represents over 110 security firms, noted: "It is the right area of emphasis if we want to professionalise the sector."

Even though there are 70,000 people qualified to work as security guards, only 33,000 actually do so. The median monthly basic pay for security guards is about $800, according to Manpower Ministry data.

The MOM, National Trades Union Congress and associations representing security firms have formed a national committee to boost the pay of security guards using a compulsory wage ladder. Under the ladder, which kicks in from September 2016, the minimum monthly basic wages of security guards will rise to $1,100