Police officers who use drastic new tactics to ram criminals off scooters could be putting their "livelihood and liberty" at risk, a federation leader has warned.
Metropolitan Police bosses emphasised the hardline approach on Friday as they revealed that moped crime has fallen by 36% in the capital year on year.
The offences have been a major issue in England and Wales, with some gangs stealing 30 phones an hour in the capital.
On Monday, chairman of the Metropolitan Police Federation Ken Marsh called for protection for officers who use "tactical contact" in a high speed chase.
He said: "I have seen nothing yet about how our colleagues will be protected when the worst happens," said Ken.
"Thankfully, in all the examples we have seen so far, there has not been a serious injury to a suspect.
"We need the senior management of the Metropolitan Police and the Independent Office for Police Conduct to please explain that when this does happen - and it will - what position my colleagues will find themselves in?
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"Will they be backed? Or will they be left to the mercy of the courts? Will their livelihood and liberty be put at risk? Because that is the reality. And our major concern."
Proposals were unveiled by the Home Office in May for greater protection for police officers involved in high speed chases with scooters.
Under the current law the same legal test for careless and dangerous driving offences is applied to police officers and the general public.
But under the Government's proposals, police drivers would be subject to a separate test when determining whether they should face action after incidents.
It would require an officer to drive "to the standard of a careful and competent police driver of a similar level of training and skill", using appropriately authorised driving tactics that are necessary and proportionate to the circumstances.
Mr Marsh said officers potentially face a lengthy investigation just by starting a high speed pursuit.
"It is a massive ask for our members. Officers have to carry out a dynamic risk assessment straight away and are having to make that split-second decision, do I pursue that individual and what are the consequences that are going to come from that for me?
"Remember they could face investigation if a suspect is injured during a pursuit - even if my colleagues do not make any contact with their bike. Just by launching the pursuit, police officers take a risk.
"There need to be protections around this afforded to our colleagues - both in law, from the force and with public, political and media opinion. They are doing nothing more than their jobs, trying to apprehend someone who, nine times out of 10, has committed a horrendous offence. They must be backed."
Force chiefs have said there is no maximum speed for police cars to hit mopeds, and that it is a common misconception among moped thieves that officers will end their pursuit if the suspect drives dangerously or removes their helmet.
Commander Amanda Pearson said: "Offenders on mopeds and motorcycles who attempt to evade the police are making a choice that puts themselves and others at risk.
"So our message is clear: We can, we will and we do target those involved in moped and motorcycle crime at every opportunity."
Officers also use DNA forensic tagging which sees suspects and their vehicles marked with a spray which can be spotted by UV lights up to one month after the crime was committed, directly linking the perpetrator to the crime.
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From January 2017 to October 2017 there were 19,455 moped-enabled crimes across London, compared with January 2018 to October 2018 when there were 12,419 offences - a fall of 36%.
An IOPC spokesman said: "The IOPC has a duty to investigate deaths or serious injuries following police contact and we do so based on whether the officers concerned followed their own force guidance and policies and current legislation as determined by Parliament.
"Police officers are entitled to use force that is reasonable and proportionate to the circumstances.
"Any decision to prosecute an officer can only be made by the Crown Prosecution Service and any decision to convict would be made by a jury or magistrate.
"Over a five-year period involving nearly 100 independent investigations into road traffic incidents involving police vehicles where a death or serious injury has occurred, two officers were prosecuted for pursuit-related offences and none were convicted."