Unions representing police officers have criticised proposals to deploy officers from their forces to help the PSNI cope with serious public disorder.
The Police Federation of England and Wales, and Scottish Police Federation described the move as "too dangerous".
The unions said it would be unfeasible for officers to be deployed on front line duties due to the "very volatile and demanding circumstances".
The PSNI said officers brought in would not be used on front line duties.
The mutual aid policy allows for officers from any force in the UK to be sent to assist members of another force.
The PSNI could use the policy if it needed help dealing with a sustained period of serious public disorder.
But the Police Federation of England and Wales, and the Scottish Police Federation, have issued a joint statement saying it would be too dangerous for their officers to work on the front line in Northern Ireland.
The statement said recruits to the PSNI have to acquire specialist skills and a 24-hour awareness built up through decades of terrorist conflict, and that this alertness is essential for survival.
The statement has been rejected by PSNI Assistant Chief Constable Duncan McCausland, who has been involved in discussions with other police forces about the policy.
ACC McCausland insisted that the PSNI has the resources it needs to cope with public disorder, and said that even if there was a need to deploy officers from other parts of the UK, they would not be used for frontline duties or counter terrorism.