Police should stop "stoking fears" around the prosecution of marksmen who carry out fatal shootings, the head of a watchdog has warned.
Chairwoman of the Independent Police Complaints Commission (IPCC) Dame Anne Owers hit back at claims that officers will not volunteer to carry guns because they fear drawn-out legal proceedings if they have to shoot a suspect.
In a fierce retort to claims that the UK faces a shortage of marksmen to deal with terrorist attacks, she said police risk "appearing to be resistant to robust investigation" in fatal cases.
The "challenging and dangerous circumstances in which firearms officers operate" cannot mean that they have "impunity" from an independent investigation into fatalities.
She said: "Our independent scrutiny should not cause any officer to be concerned about taking on a firearms role. Those within the police service should be careful about stoking such fears, and of appearing to be resistant to robust investigation when it actually happens.
"It is in all our interests, and those of the public, that we are able to obtain best evidence as quickly as possible, and the cooperation of officers and the service is crucial in this."
The IPCC has only used its powers of arrest once in 12 years of investigating 29 fatal police shootings - in the case of Jermaine Baker, who died in north London in December 2015.
Dame Anne said this case should not be used to question what the watchdog would do in the case of a terrorist attack, citing the murder of Lee Rigby when the armed officers who shot his two killers were found to have acted legitimately.
"It is very disappointing that some in the police service, without benefit of our evidence, are using a single case to cast doubt on our potential actions in a major terrorist incident. The test in all such situations is what officers genuinely believed, given the circumstances.
"We are well aware that, in the event of a marauding terrorist attack, firearms officers will be on the front line, making split-second decisions to protect the public, and our approach to investigation will clearly have to take account of the realities on the ground."
Chairman of the Police Federation Steve White, who is due to speak at the group's annual conference on Tuesday, warned that Britain has been left vulnerable to a terror attack due to a shortage of police marksmen.
He told the Press Association: "Officers don't want to carry firearms because they are concerned that if they discharge it, they are going to get arrested for murder.
"The Government has got to provide a message to the service, not that it has a get out of jail free card, but that officers will be supported appropriately and fairly in order to encourage officers to step forward and take on additional responsibility."
Plans were put in place to train an extra 1,500 armed officers in the wake of the terrorist atrocities in Paris, but Mr White warned that forces were struggling to fill the vacancies, for which officers have to volunteer.
Deputy Chief Constable Simon Chesterman, the national lead for firearms, also admitted to The Guardian: "We might have a challenge attracting the right number of volunteers and retraining the people we have got." He added: "It is not a crisis but it might become one."
A Government review is being carried out into the law around police use of firearms and rules for post-shooting in investigations.
Home Office figures released in April revealed that the number of trained marksmen had dropped by a fifth in the past seven years - down by 1,259 to a current total of 5,647.
On Tuesday, chairman of Hampshire Police Federation John Apter claimed a national shortage meant unarmed officers in rural areas would be "sitting ducks" in a terrorist attack.
He told BBC Radio 4: "Being realistic, if a firearms unit was coming from the middle of the county you are still talking about 30 miles away - you are not talking about a few minutes.
"So the only officers that you have available are unarmed and vulnerable officers and they are the officers that are saying to me that in a terrorist situation they would be sitting ducks."