Police officers directly involved in fatal incidents should be separated as quickly as possible to prevent conferring, a watchdog has said.
The step is part of new guidance from the Independent Police Complaints Commission (IPCC).
It relates to how evidence should be collected immediately after a member of the public has died or been seriously injured during contact with the police.
But the Police Federation said the move was "without cause".
The guidance proposes that key policing witnesses should be separated as soon as it is "operationally safe" until after they have provided their personal initial account.
The IPCC said separating officers after an incident to prevent conferring was designed to ensure officers provide individual accounts of only what they saw, heard and did.
This avoids actual or perceived collusion or their accounts being unintentionally influenced by those of others, the watchdog said.
The issue of conferring among officers came under the spotlight following the fatal shooting of Mark Duggan in 2011. A jury later concluded he was lawfully killed by police.
IPCC deputy chair Sarah Green said: "The measures we have outlined do not treat police officers as suspects, but as witnesses whose early individual accounts will help ensure the integrity and smooth running of the critical early stages in any investigation."
Che Donald, firearms lead for the Police Federation of England and Wales, said officers were "under no illusion" of the scrutiny they face following a death or serious incident.
He added: "They are witnesses first and foremost and to separate them in the immediate aftermath of a highly traumatic incident is neither proportionate nor necessary and without cause."
If approved by the home secretary, all police forces in England and Wales would be obliged to make use of the new guidance in the event of fatalities or serious injuries resulting from firearms operations, incidents in custody or other police contact.