“Significant questions” remain about the behaviour of Pc Simon Harwood, who was cleared of the manslaughter of Ian Tomlinson during the G20 protests, the police watchdog said.

The Independent Police Complaints Commission’s deputy chairwoman, Deborah Glass, ordered that Mr Harwood will face public Metropolitan Police disciplinary proceedings.

Mr Harwood, 45, was acquitted at Southwark Crown Court on Thursday of the manslaughter of Mr Tomlinson in April 2009.

Outside court stepson Paul King, of the Isle of Dogs, called the verdict “a joke”, vowing to pursue the issue in the civil courts.

Mr Harwood hit 47-year-old Mr Tomlinson with a baton and pushed him to the ground during the G20 protests. The father-of-nine walked 75 yards before he collapsed, and died later in hospital from internal injuries.

Mr Harwood faced a string of allegations about his behaviour before he came across Mr Tomlinson.

Ms Glass said: “It is clear that significant questions remain in connection with his actions on the day Ian Tomlinson died.

“Whether or not those actions were reasonable will be tested further at a misconduct hearing in September.

“There are also questions that the Metropolitan Police Service must answer.

“Pc Harwood was able to retire while facing disciplinary proceedings for previous alleged misconduct towards a member of the public.

“That he was then re-employed by the force, first in a civilian role and later as a constable, is simply staggering.”

At an inquest into Mr Tomlinson’s death last year the jury returned a verdict of unlawful killing.

Speaking outside court on Thursday, Mr King said: “After the verdict at the inquest last year, we were expecting to hear a guilty verdict. It really hurts.

“There has to be one final answer to the question ‘who killed Ian Tomlinson?’ That will now be pursued in the civil courts.”