About 15,000 off-duty police officers plan to march through central London on Wednesday to show their anger at the government's decision not to backdate a pay rise.

|PIC1|The row erupted in December when Home Secretary Jacqui Smith announced that a 2.5 percent pay increase, agreed by an independent tribunal, would not be backdated to September as expected.

The government argued that this had been done to keep a lid on public spending.

But outraged police officers said this effectively cut the rise to 1.9 percent, less than the rate of inflation and saving the government 30 million pounds.

The Police Federation, a body which represents 140,000 officers in England and Wales, responded by calling for Smith to resign and saying it would ballot its members on whether to overturn a ban on strike action.

"We appreciate that for many of you the march, rally and lobby is the focal point of your anger and frustration after the government has failed to honour the independent arbiters' decision in full," the federation said in a message to members.

"The Police Federation is very grateful to you for giving up your free time to take part."

"That action alone demonstrates to the media, politicians and public the strength of feeling amongst police officers who have been betrayed by this government."

The protesting officers will begin their march at Hyde Park at about 11.30 a.m., stopping off at the Home Office to deliver petitions from every force in England and Wales, and finishing at the Tate Gallery. Rallies will then be held in Westminster.

Those taking part have been instructed to behave in a dignified and controlled manner amid suggestions that groups with grudges against the police might try to stage rival protests.

Since the dispute began, Prime Minister Gordon Brown has insisted that the government would not back down, saying the pay increase was in line with other public sector deals and had been made in the national interest.

But police say Smith's decision was the first time a Home Secretary had failed to ratify a police pay arbitration award.

The Police Federation plans to ballot officers next month over possible industrial action, banned under laws introduced in the 1990s.

"For nearly a century that is what has set officers apart from all other workers in the public sector," the Federation said. "If the government wish to treat us like all other workers, then perhaps we should have the same employment rights."

On Monday, a left-wing think tank said police pay should be overhauled to reflect performance and skills, rather than simply length of service.

The Institute for Public Policy Research said the current system failed to reward those doing the most difficult or dangerous duties, but the Police Federation said the recommendation was impractical.