Blunkett complained juries did not represent society

Along with judges, politicians, vicars, doctors and midwives, police have had their exemption from juries removed under last year's Criminal Justice Act.

Ministers hope the change will increase jury representation from these groups.

But the Met Police Federation, which represents rank-and-file officers in the capital, says it is unworkable.

The idea of having policemen on jury service for long periods will detract from the protection of the public David Davis Shadow home secretary

He said there was an added problem that police officers could know defendants in the case.

The changes to the Act, which cover police in England and Wales, came into force on 5 April.

But Mr Smyth said his organisation only found out about them last month when an officer received a call to attend court and was unable to secure an exemption, he said.

His concerns were echoed by shadow home secretary David Davis, who told The Sun: "We do not have enough police on the streets as it stands.

"The idea of having policemen on jury service for long periods will detract from the protection of the public."

'Dodging'

The old system was criticised for being too easy for some professions to seek exemptions, leaving juries heavily reliant on people not working.

In 2001, Mr Blunkett said: "Anyone who shirks the responsibility of jury duty helps to undermine the foundation of one of our country's most cherished freedoms."

A spokesman for the Department for Constitutional Affairs said only 200,000 of the 480,000 people called up for jury duty each year were eligible to serve under the old rules.

"Jury service is an extremely important civic duty. For too long a large number of people have been excused from serving on a jury because of their profession or position in society."