The Home Secretary was asked to intervene in a fresh police race row today as it was claimed at least 20 forces blocked an inquiry into discrimination against Muslim officers.

Barely half of the 43 forces in England and Wales co-operated in an audit into the treatment of officers from ethnic minorities, the National Association of Muslim Police said.

In a letter to Jacqui Smith, Zaheer Ahmad, president of the association, also claimed information from the forces that did respond suggested routine racial harassment.

Home Secretary Jacqui Smith has been urded to order a 'critical review' of racism in the police

Referring to 10 years since the inquiry into the murder of Stephen Lawrence found the Metropolitan Police to be 'institutionally racist', an association spokesman said its audit revealed 'we still do not have the level of diversity that the public rightly expects.'

Progress on diversity was 'painfully slow', he added.

The revelations come just days after it emerged Tarique Ghaffur, Britain's most senior Muslim officer, was considering suing the Metropolitan police for discrimination.

From the data provided to the association, it was revealed 95% of black and minority ethic officers were in the most junior rank.

Figures also revealed Muslim officers were almost 'entirely absent' from specialist operations such as counter-terrorism - a move Mr Ahmad believed was hampering the fight against terrorism.

In the questionnaire, the association asked forces about the promotional prospects, rank and number of Muslim and black officers employed.

Initially just 11 of the 43 forces replied - 23 co-operated when the deadline was extended.

In his letter sent to Ms Smith and all the chief constables, Mr Ahmad asked: 'Why were some forces unable or unwilling to cooperate while others completed in full and on time? Why did some forces refuse to complete on grounds of the pretext of the data protection act, while others said they did not have the time to take part?

'If the police are serious about ensuring that Muslim officers are able to rise through the ranks at the same speed as their fellow white officers, and ensuring that Muslims are deployed to counter-terrorism duties at a time of heightened national security, we must have reliable data to track progress and measure success.'

The letter, which came to light after it was leaked to the Observer newspaper, went on to warn forces the association would lodge a Freedom of Information request if they continued to refuse to take part.

It also urged the Home Secretary to order a 'critical review' of racism in the police before the 10-year anniversary of Sir William Macpherson's report.

The letter continued: 'Muslim officers are being overlooked, potentially discriminated against in promotion or are failing to receive the necessary training and personal development that would allow them to rise through the ranks.'

From the replies it received, just 0.3% of police officers were found to be Muslim, compared with three per cent of the British population.