The kettling tactic used by police to pen in 5,000 people during the G20 protests carries significant risks, the man leading the review of public order policing said today.

Denis O’Connor, HM Inspector of Constabulary, also condemned as "utterly unacceptable" the removal by some officers of their police numbers.

Mr O’Connor said his inquiry would cover industrial disputes as well as marches and protest events such as climate camps.

“There are risks with kettling,” he the Commons Home Affairs Select Committee. “If you are trapped there on your way to work, if you are pregnant, if you are elderly, if you haven’t had anything to drink or eat and you are contained, imprisoned almost, for a period of hours, then very difficult consequences can follow.”

Future use of the kettle may require greater flexibility, possibly with points in police cordons where people could leave, thus alleviating risks.

Mr O’Connor added that the tactic had been introduced to prevent a repeat of situations in London where demonstrators ran through the streets causing trouble.

He added: “In the past some people were allowed to run through Regent Street and break windows and terrorise people in the shops. So we’re in a bit of a dilemma.”

The new head of the police inspectorate has been asked by Sir Paul Stephenson, the Metropolitan Police Commissioner, to review police tactics at the G20 and other demonstrations.

Kettling has been in use for several years and was declared legal by the House of Lords this year after a series of court challenges over the holding of demonstrators in Oxford Circus on Mayday 2001.

Mr O’Connor intends to have an interim report completed by June with his full review to be published in September.

His inquiry will also examine the issue of officers removing or covering their numbers and consider whether they should have to go as far as wearing name tags.

Mr O’Connor said: “For people not to be wearing their numerals is utterly unacceptable. That’s it. It’s not a long-winded thing, it is something that is very clear cut. I am very concerned with that issue and I firmly hope it will be rectified with some certainty.”

The inspectorate hopes to contact some of the people who have contacted the Independent Police Complaints Commission (IPCC) about police tactics.

The IPCC will concentrate on individual incidents of alleged misconduct by officers and is running three investigations including those into the death of Ian Tomlinson, a newspaper vendor, on April 1 and an alleged assault on Nicky Fisher, a protester, on April 2.

Nick Hardwick, chairman of the IPCC, later told the committee that he was deeply concerned about the issue of officers not wearing their identification numbers.

He said: “In my view, police officers have an absolute obligation to have their identity on view at all times. Their superiors have an obligation to make sure that is complied with. I’m absolutely clear that officers should be required to be identified at all times. We would treat that as a potential disciplinary offence if they do not."

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