Plans to prosecute anyone found carrying a knife in parts of Birmingham have been condemned as a gimmick after the Home Office admitted it could not explain when, where or how the scheme would operate.

Gordon Brown, the Prime Minister, revealed plans to prevent police cautioning people carrying knives or guns.

Zero-tolerance will be applied in 12 knife crime hotspots, mainly in large cities like Birmingham, London, Manchester or Liverpool.

But the Home Office said it could not reveal where these hotspots were, or when the change in policy would come into effect.

Officials admitted the Government did not have the power to force police to prosecute offenders instead of issuing cautions.

A decision on whether to prosecute or not is made by individual forces. However, ministers intend to encourage the Association of Chief Police Officers and the Crown Prosecution Service to agree to increase the number of prosecutions.

Birmingham MP Andrew Mitchell (Con Sutton Coldfield) said: "This is yet another example of Labour's failure to bring about change. "Gordon Brown is desperate to give the opposite impression, but he is still in the age of spin.

"Once you strip away all those words, there is precious little in what he has said that will make our streets safer."

He was responding to comments from the Prime Minister who said: "We have to send out a message and reinforce it with action.

"It is completely unacceptable to carry a knife or a gun. Where police have previously been cautioning people, there now has to be a presumption of prosecution."

He also expressed worries about violent computer games featuring knives.

"No one wants censorship or an interfering state," he said.

"But the industry has some responsibility to society and needs to exercise that."

His intervention follows rising concern about knife crime on British streets.

Mr Brown added: "You cannot be casual or cool about knives."