Police support workers have been offered a pay rise of 2.5%, backdated to September, the same deal being fought for by police officers.
Community support officers have been offered a backdated rise
The offer, to 71,000 staff in England and Wales, include those working as custody, forensics and police community support officers.
The 2.5% deal offered to police officers is due to begin in December.
A Commons home affairs select committee said the rise for police in England and Wales should also be backdated.
The government has agreed to the rise but not to backdate it to September, saying this meets its inflation target.
Ben Priestley of Unison said this year's police staff pay negotiations have been undertaken "within an increasingly challenging political and policy environment".
He said: "Under these circumstances, we are putting the 2.5% offer to members as the best achievable by negotiation.
"At the same time, we call upon the home secretary to do the right thing and honour in full the 2.5% awarded to police officers as a result of their arbitration.
"By treating police officers and police staff pay differently, the government risks undoing all the good work over the last 10 years to build a unified police service.
"Police staff and police officers work side by side to protect our communities - they do not want to be divided over pay."
Shadow Home Secretary, David Davis, said the backdating of pay for police support workers "adds insult to injury".
He said: "Not only has the home secretary been incompetent and less than straight forward with the police, her claim that restraint was required in the police pay deal has know been completely undermined."
A Home Office spokeswoman said: "The Police Staff Council is responsible for setting the pay of police staff including Police Community Support Officers.
"The home secretary has no statutory role in making that decision.
"The Home Office does not support the Police Staff Council employer side offer, as it will not produce an outcome that is consistent with affordability and UK government pay policy."
Meanwhile, the Commons select committee's chairman Keith Vaz MP has written to Home Secretary Jacqui Smith urging her to "honour the terms of the independent tribunal" which called for a 2.5% rise.
Some 177 MPs have signed parliamentary motions saying the award should be paid in full to England and Wales officers.
The 2.5% pay deal was decided through the independent Police Arbitration Tribunal.
It will see all police constables paid a minimum of £21,500, while those with the longest service will receive £33,800.
But officers say if it is introduced this month and not backdated to September, an entry level police constable will lose £131, and a sergeant will lose £206.
The Scottish Government has agreed to implement the tribunal's award in full.
The police are forbidden from taking strike action by an act of Parliament, the Police Federation will ballot in the New Year over a demand that the ban is removed.