Video of police attack on Ian Tomlinson. guardian.co.uk

The officer caught on video assaulting Ian Tomlinson minutes before he died faces being dismissed from the Metropolitan police before Christmas, the Guardian has learned.

But in an unusual alliance, the Tomlinson family and supporters of the officer, PC Simon Harwood, say court action should be considered to stop a police disciplinary hearing going ahead before an inquest into Tomlinson's death has taken place.

Prosecutors have already said PC Harwood assaulted Tomlinson but they were unable to bring criminal charges. The Tomlinson family were angered by that decision and believe an inquest may be their best and final hope of gaining justice and holding the police to account.

Video evidence of PC Harwood striking Tomlinson with a baton and shoving him to the ground will feature in the Met case against him. The incident took place at last year's G20 protests in central London.

Sources say the officer will appear before a police disciplinary board within months, charged with gross misconduct which carries a maximum penalty of him being sacked from the force.

But the inquest into Tomlinson's death is not due to start until May 2011 at the earliest, and Met bosses believe that is too long to wait before they can exercise their right to hold a disciplinary hearing into their own officer.

A senior police source said: "We don't want to wait that long. We want to show we are taking the issue seriously and not dragging our feet."

Police discipline rules are complex and notoriously slow. The senior source said: "Would any other business be held back from doing their own internal discipline because of wider external factors?"

The source added that Harwood was still on full pay and has been suspended from duty since April 2009: "It's not in the public interest for someone to be a police officer for more time than they need to be."

Once the Met announces its decision to hold the discipline hearing, the other parties will be written to and have 30 days to respond and give their views. The Met, says the senior source, is anticipating a legal challenge.

One factor for the Met's decision to hold the hearing before the inquest is that it could be less damaging for its reputation if Harwood appears at the inquest as a former officer.

That decision has put the police and the Tomlinson family on a fresh collision course. Tomlinson's son, Paul King said: "I can see how it would look nice for the police if the officer turned up to the inquest in his civilian clothes."

The inquest is expected to be enormously difficult for Harwood and the force, and will see him called to face questions in public about his conduct.

Police officers who have been helping PC Harwood are also angered by the proposed action of their bosses.

A supporter of PC Harwood said: "This will be unfair on the officer. It's unfair to put him before the [discipline] board before the inquest.

" The proper place for the facts to come out is the inquest. It is very difficult for him to give a full account at a discipline board. His legal team will have to seriously consider a challenge."

Jules Carey, solicitors for the Tomlinson family said: "If the Metropolitan Police are determined to hold misconduct proceedings before the inquest has determined the cause of Ian Tomlinson's death, then they are effectively ruling out the possibility of the officer facing misconduct charges relating to his death."

"Clearly I shall need to advise Ian's family about their legal options to challenge any decision by the MPS to proceed with misconduct proceedings ahead of inquest."

In July the Crown Prosecution Service announced PC Harwood could not face criminal charges because it believed clashing medical evidence made it unlikely that a jury would convict him of manslaughter.

The CPS has said it would reconsider the decision not to prosecute after the results of the inquest in case any new evidence emerges.

One pathologist said Tomlinson, 47, died from a heart attack, but two other pathologists later separately concluded that he had died of internal bleeding as a result of blunt force trauma.

The CPS's view clashes with that of an investigation by the Independent Police Complaints Commission, which concluded there was sufficient evidence to charge the officer with manslaughter.