Gordon Brown is to set out plans to restore trust in politics and make the Government more accountable to Parliament.
At his second meeting of the Cabinet in less than two days as Prime Minister, he will hold a "lengthy discussion" on his constitutional reform agenda.
A senior ally, new Justice Secretary Jack Straw, indicated that the moves would involve redressing the balance of power between the Government and MPs.
Mr Brown will make good on his commitment to give Parliament a say in declarations of war and take intelligence assessments out of the hands of press officers.
The plans, trailed before Mr Brown became Premier, will be seen as a symbolic break with the Tony Blair era dominated by the controversy over Iraq.
Mr Straw said: "There isn't any doubt that one of the reasons why trust in politicians and the Government has been reduced is because of what happened over Iraq."
It was a matter of "profound regret" that intelligence about Saddam Hussein's alleged weapons of mass destruction turned out to be inaccurate, he said.
Mr Brown held his first meeting of his new Cabinet on Thursday, following a sweeping reshuffle.
He is due to make more junior ministerial appointments.
In a more substantial meeting than Thursday's 45-minute session, Mr Brown will outline to Cabinet colleagues "the conduct of Government and the conduct of ministers", his spokesman said.
