In an open letter published on Sunday, Sir Mark Rowley said his officers needed "sufficient legal protection to enable them to do their job and keep the public safe, and the confidence that it will be applied consistently and without fear or favour".

It comes as the Ministry of Defence placed the army on standby to support the force after dozens of officers handed in their weapons after a fellow marksman was charged with the murder of 24-year-old Chris Kaba in Streatham last year.

Home Secretary Suella Braverman has since launched a review "to ensure [officers] have the confidence to do their jobs".

She said officers "mustn't fear ending up in the dock for carrying out their duties"

Sir Mark has now called for a reform of the law to change the threshold by which they can be held criminally responsible for using firearms.

Progress, he said, to deliver change in the Met was "undermined by a system not set up to help officers succeed".

The force responds to around 4,000 armed incidents every year, with officers discharging firearms on "less than two occasions", it said.

Sir Mark said: "There is a concern on the part of firearms officers that even if they stick to the tactics and training they have been given, they will face years of protracted legal proceedings which impact on their personal wellbeing and that of their family.

"While previous reviews have been announced, they have not delivered change.

"Carrying a firearm is voluntary. We rely on officers who are willing to put themselves at risk on a daily basis to protect the public from dangerous criminals including terrorists."

He added: "I make no comment on any ongoing matters that are sub judice but the issues raised in this letter go back further."

Legal proceedings 'sap confidence' of police to act

Sir Mark said when someone loses their life it is "clearly devastating for the family" but cited a number of examples of officers facing protracted legal proceedings.

In December 2018 two officers fired shots at a "serious criminal who was part of a dangerous gang responsible for armed robberies across London".

Two years later, the officers were charged and appeared before a court, and then spend a further year on bail.

"When the case finally got to trial the CPS offered no evidence, accepting that there was no realistic prospect of conviction and they were cleared. It cannot be right that these officers were put through this turmoil and had their lives put on hold for so long. Even now they are still waiting for a misconduct hearing," Sir Mark said.

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He said in cases where officers do act improperly, the system "needs to move swiftly and assertively to deal with them, rather than tying itself in knots pursuing good officers".

This, he said, "saps the confidence of all officers to act against criminals".

Officer appears in court

A Metropolitan Police officer, named only as NX121, appeared in court on Thursday in connection with the fatal shooting of Mr Kaba.