Tony Blair is to set out the distinct role of police and security services in Northern Ireland, in a written statement to the Commons on Wednesday.
The move follows intensive negotiations between the government and Sinn Fein on policing over the Christmas period.
It is believed the government hopes the statement will reassure nationalists about MI5's future role.
Sinn Fein sources believe it may answer nationalist doubts about interference by the security services in policing.
Republican sources suggest it may mean an end to any suggestion of secondments between the police and MI5.
There are also suggestions that a new Policing Board sub-committee will be set up to deal with security.
Both the SDLP and Sinn Fein have raised questions about oversight of the security services.
MI5 is building a £20m headquarters in Palace Barracks outside Belfast.
Sinn Fein's Gerry Kelly said his party was awaiting with interest the prime minister's statement .
"This statement must put in place mechanisms to ensure the complete separation of MI5 from civic policing," he said.
"There can be no integration of MI5 and the PSNI."
The SDLP have expressed concern that settling for an apparent separation of MI5 would diminish any likelihood of the Police Ombudsman being able to hold the security service to account.
BBC Northern Ireland political editor Mark Devenport said while the details of the statement have not been disclosed it is thought it may confirm a monitoring role over MI5 for the Liberal Democrat peer Lord Alex Carlile who currently oversees anti-terror laws.
Sorting out policing is critical to restoring devolution.
Fresh powers
Meanwhile, it emerged in the assembly on Monday that police in Northern Ireland are to be given fresh powers to seize documents and files in order to tackle serious crime.
Detectives could seize material without any explicit reason to suspect crime. It is understood similar laws are not planned for Great Britain.
Security minister Paul Goggins said searches would not be "willy-nilly".
SDLP MLA Alex Attwood said that at a time when other security measures are being relaxed, more stringent police powers should not be introduced.
"How can you on the one hand normalise Northern Ireland society and on the other hand give the police this enormous power?" he asked on the BBC's Good Morning Ulster.
"(Officers could) go into any building under a lawful search and seize any document, even if there is no suspicion of that document being relevant to crime."
The assembly's policing and justice committee debated the issue on Monday.
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