The police watchdog will investigate how two forces handled the release of information to the media after the death of Ian Tomlinson at the G20 demonstrations.
Mr Tomlinson's family have complained that the Metropolitan Police knew its officers had contact with the newspaper seller before he collapsed and died during the protests on April 1.
They allege that the force issued "misinformation" about the level of contact until a newspaper published a video six days later showing an officer pushing Mr Tomlinson to the ground and hitting him with his baton.
Scotland Yard's first statement about 47-year-old Mr Tomlinson's death was issued late on April 1. It said officers tried to revive him after being alerted to his collapse by a member of the public, and suggested that some of the protesters hampered their efforts.
The statement read: "A member of the public went to a police officer on a cordon in Birchin Lane, junction with Cornhill, to say that there was a man who had collapsed round the corner.
"That officer sent two police medics through the cordon line and into St Michael's Alley where they found a man who had stopped breathing."
It went on: "The officers took the decision to move him as, during this time, a number of missiles - believed to be bottles - were being thrown at them."
But on April 7 the Guardian newspaper released a video shot by a fund manager from New York which showed Mr Tomlinson being shoved to the ground from behind by a helmeted police officer.
When the footage was slowed down, the officer could be seen apparently using his baton to hit the newspaper vendor on the leg.
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The Independent Police Complaints Commission (IPCC) said its inquiry would be independent and look at how both the Met and City of London Police handled the media. It will be separate from the IPCC's ongoing investigation into the circumstances of Mr Tomlinson's death.
