A British police officer has been questioned on suspicion of manslaughter over the death of a man in protests during the London G20 summit earlier this month.

Britain's police watchdog said the action had been taken against the unnamed officer after a second post-mortem found that newspaper seller Ian Tomlinson had not died from a heart attack as first thought.

Mr Tomlinson, 47, died after being caught up in protests near the Bank of England in the heart of London's financial district as he made his way home on April 1, the day before world leaders gathered to discuss the economic crisis.

Video footage taken by a New York Fund manager showed Mr Tomlinson, who had not taken part in the demonstrations, being shoved to the ground by a police officer in riot gear. He collapsed shortly afterwards in a nearby street.

The officer involved has already been suspended by London's Metropolitan Police and the matter is being investigated by the Independent Police Complaints Commission.

"Following the initial results of the second post mortem, a Metropolitan Police Officer has been interviewed under caution for the offence of manslaughter as part of an on-going inquiry into the death of Ian Tomlinson," the IPCC said in a statement.

The first post mortem had recorded that Mr Tomlinson had died from natural causes. But lawyers for Mr Tomlinson's family said yesterday a second pathology report had indicated that the cause of death was abdominal haemorrhage.

"First we were told there had been no contact with the police, then we were told that he died of a heart attack," said Paul King, speaking on behalf of the family. "As time goes on we hope the full truth about how Ian died will be made known."

London's police force said it could not comment while the IPCC probe was ongoing but said it would fully cooperate.

The watchdog has received a total of 145 complaints about the policing of the protests which turned increasingly violent as the day wore on, with confrontations between anti-capitalist demonstrators, environmental campaigners and riot officers.

The capital's police chief has expressed his concern about the video images and has ordered a review of public order tactics, especially the use of "kettling" where protesters are herded by officers into a confined space.

That followed growing criticism from politicians and civil rights groups that the police had used excessively violent force to deal with the protests.