Thousands of police officers have second jobs, many in defiance of official guidelines, prompting concerns over a “part-time police service”.

Chief constables are allowing officers to work as private investigators, security instructors and late-night drivers, even though these posts are seen as “incompatible” with officers’ primary work.

A parliamentary committee pledged yesterday to investigate potential conflicts of interest after the number of officers taking on extra work increased starkly in the past 18 months.

Official estimates suggest that 15,000 officers have declared additional work or business interests. But a Freedom of Information (FoI) request by The Times has found that the numbers doing so had tripled in some forces since October 2009.

The Labour MP Keith Vaz, Chairman of the Home Affairs Select Committee, said he had been shocked to learn that work was allowed in private investigation. “The issue of conflict of interest is of very serious concern,” he said. “The public will find it most unsatisfactory that police officers have second jobs. It’s a very important profession. We simply don’t want a part-time police service.” He called for a national directive on outside work.

The Police Federation, which represents forces in England and Wales, said that many officers had to take second jobs to make ends meet in a climate of cuts and pay freezes, but warned that officers could be over-tired at work.

Although individual chief constables take ultimate responsibility for the approval of secondary jobs, the Association of Chief Police Officers (Acpo) has issued guidance highlighting roles that should be ruled out. Driving jobs are seen as problematic as they “may interfere with … [the recommended] 11 hours’ break between working shifts”, while working as a driving instructor could lead to “poor attendance levels”, Acpo said.

All but two forces in England and Wales, however, allow officers to take on such work, from long-haul trips in HGVs to taxi-driving and giving driving lessons. In Bedfordshire Police, driving-related jobs made up more than 30 per cent of secondary interests.

Although the Acpo guidelines also warn against officers using “specialist skills obtained through the police” — such as Taser instructing — for external interest, there were examples that could lead to conflict.

Of the 3,671 Metropolitan Police officers who have declared extra work or business interests, 14 are working in the security sector. Gloucestershire Constabulary staff are allowed to work in insurance investigations, and the South Yorkshire force tabled one business interest as an “officer in criminal justice admin/dept”.

In Surrey the extra work includes “scene management and investigative skills training”. An officer in Cambridgeshire provides “security solutions”. Warwickshire Police has allowed training of security guards, and one officer declared a counter-fraud investigation company, although business searches yielded no results in its name.

Her Majesty’s Inspectorate of Constabulary has found officers working in Taser training and as instructors in personal protection equipment instruction. This was impossible to discern from the FoI request as most forces gave scant detail of officers’ extra-curricular interests. In many cases they were labelled simply as “training”. A significant proportion of the declarations were for property interests.

The inspectorate has highlighted variations in the way the guidance on second jobs and business interests was enforced, declaring that there were “indefensible inconsistencies”. It expressed concern that there was not enough monitoring of what staff were doing in their spare time.

An Acpo spokeswoman said: “The service will now closely examine the case for national standards.”

A wide variety of jobs are considered appropriate by Acpo. They include massage therapy; pall bearing; work as mystery shoppers; etiquette training; and diet counselling.

Paul McKeever, chairman of the Police Federation, said that officers were faced with pay cuts and increased pension contributions. “Very few can take that sort of hit,” he said. “They have to look for additional income elsewhere. That is going to cause many officers who wouldn’t have previously done so to try and find some sort of secondary income.”