CAIRO — The military council governing Egypt is moving to lay down ground rules for a new constitution that would protect and potentially expand its own authority indefinitely, possibly circumscribing the power of future elected officials.

The military announced Tuesday that it planned to adopt a “declaration of basic principles” to govern the drafting of a constitution, and liberals here initially welcomed the move as a concession to their demand for a Bill of Rights-style guarantee of civil liberties that would limit the potential repercussions of an Islamist victory at the polls.

But legal experts enlisted by the military to write the declaration say that it will spell out the armed forces’ role in the civilian government, potentially shielding the defense budget from public or parliamentary scrutiny and protecting the military’s vast economic interests. Proposals under consideration would give the military a broad mandate to intercede in Egyptian politics to protect national unity or the secular character of the state. A top general publicly suggested such a role, according to a report last month in the Egyptian newspaper Al- Masry Al- Youm. The military plans to adopt the document on its own, before any election, referendum or constitution sets up a civilian authority, said Mohamed Nour Farahat, a law professor working on the declaration. That would represent an about-face for a force that, after helping to oust President Hosni Mubarak five months ago, consistently pledged to turn over power to elected officials who would draft a constitution. Though the proposed declaration might protect liberals from an Islamist-dominated constitution, it could also limit democracy by shielding the military from full civilian control.

The military is long accustomed to virtual autonomy. Its budget has never been disclosed to Parliament, and its operations extend into commercial businesses like hotels, consumer electronics, bottled water and car manufacturing.