LONDON - Police pay structures should be reformed to reflect performance and skills rather than simply length of service, a think tank report said on Monday.

Just days before off-duty officers are due to protest about their latest pay award, the Institute for Public Policy Research (IPPR) said the current set-up failed to reward those doing the most difficult or dangerous duties.

"This system does not reward expertise and discourages officers from developing much-needed specialist skills, such as tackling violent and gang-related crime," the report said.

The report comes in a week when the Police Federation, which represents 140,000 rank and file officers in England and Wales, stages a march to protest at the government's decision not to backdate a 2.5 percent pay rise recommended by an independent tribunal.

The Federation has passed a vote of no confidence in Home Secretary Jacqui Smith and is to ballot its members on whether to overturn a ban on taking strike action.

In its study, the IPPR said officers received a rise of between two and six percent on top of the national pay during their first 10 years of service.

It said there should be pay bands for each rank, with more money for those with specialist skills, with an end to incremental rises for length of service apart from in the first few years of service.

"We all know that the police do a difficult and challenging job but no system of pay is fair that rewards people solely on the basis of time served rather than their ability to do the job effectively," said Guy Lodge, IPPR senior research fellow.

"The current row over pay levels is preventing much-needed debate about how we reward police officers and how we deliver a high-performing police service."

The left-leaning IPPR said that although crime had fallen since 1997, police performance had not improved and that an increase in police funding had not led to greater productivity in terms of crime detection.

Jan Berry, the Police Federation's chairman, said it would be hard to judge what constituted police productivity.

"It's really to difficult to define in a team which part of that team - the investigator, the interviewer, the intelligence gatherer, the patrol officer - where do you start putting the value on what you value more than the others?" she told BBC TV.

"If this was a simple thing to do, we would have thought about it ages ago."