Mr Tomlinson's relatives reacted with delight after Keir Starmer, QC, said there was now a realistic chance of a manslaughter prosecution against PC Simon Harwood.
Mr Starmer last year ruled out charges despite video footage clearly showing Mr Tomlinson being shoved to the ground.
The prosecutor yesterday explained his change of mind by insisting he had reviewed new medical evidence. He also took into account an unlawful killing verdict by an inquest jury earlier this month.
PC Harwood, a father of two, from Carshalton, Surrey, will appear before City of Westminster Magistrates' Court on 20 June.
Mr Tomlinson, 47, a homeless newspaper seller, collapsed and died on the fringes of the demonstrations in central London on 1 April, 2009.
Speaking after meeting Mr Starmer yesterday, his stepson Paul King said relatives were "really happy" as a court summons was issued against PC Harwood. "We are a big step closer to where we want to be," said Mr King, flanked by Mr Tomlinson's widow. "We should have been here two years ago, but we are more than happy."
Mr Starmer said "matters have moved on in two ways" since his previous decision not to pursue criminal charges against the officer. New medical evidence at the inquest and the opinions of experts during the hearing helped change his mind, he said.
Delivering his statement at the Crown Prosecution Service's headquarters in London, he said: "The difficulties that would now confront any prosecution have changed in nature and scale from last year when a decision was taken not to prosecute, although it is clear that real difficulties remain.
"Taking the evidence as it now stands, we have concluded that, even with those remaining difficulties, there is sufficient evidence to provide a realistic prospect of successfully prosecuting PC Simon Harwood for the manslaughter of Mr Tomlinson.
"That being the case, it is clearly in the public interest that criminal proceedings be brought. Accordingly, a summons charging PC Harwood with the manslaughter of Mr Tomlinson has been obtained ."Metropolitan Police Commissioner Sir Paul Stephenson said: "This is clearly a very, very serious matter for all concerned."
He added: "I have got to be very, very concerned at an inquest verdict that returns a verdict of unlawful killing involving, as the inquest did, one of my police officers.
"My sympathies continue to be with the Tomlinson family."
The death became an international controversy after New York businessman Christopher La Jaunie handed footage he had taken of a police confrontation to the Guardian newspaper.
The DPP launched a "thorough" review after the inquest's unlawful killing verdict.
CCTV images, police helicopter footage and hand-held video recordings show Mr Tomlinson cutting a lonely figure as he staggers away from a police cordon after being hit with a baton.
Footage shows Mr Tomlinson gesturing to police and appearing angry after being sent tumbling to the ground.
His widow Julia said the death had devastated her family, adding: "I remember feeling he was the best thing that ever happened to me."