Egypt's Muslim Brotherhood, leading the field in parliamentary elections, said on Thursday it would shun a new council set up by military rulers to help oversee the drafting of a constitution.

The popular Islamist group said it feared the civilian body, which brings together politicians, presidential candidates and youth representatives, would usurp the authority of the new parliament and could become a permanent fixture in Egypt.

After participating in the initial negotiations ... it became apparent that the advisory group will have a mandate beyond the transitional period headed by the military council, said Mohamed al-Katatni, a senior member of the Muslim Brotherhood's Freedom and Justice Party (FJP).

Analysts say the advisory group could become another means for the military to bypass parliament, which looks certain to have a large Islamist bloc, after it cedes power following presidential elections next June.

The army took charge in February after the ousting of President Hosni Mubarak and on Thursday unveiled the 30-member council which will have a mandate to cover discussions on international treaties and the constitution.

The advisory council will assist the military council in all matters important to the nation and public opinion, the army council said, denying suggestions that it would continue to function once a new president was elected in June 2012.

A source close to the project told Reuters earlier that if the committee proved successful, its mandate could be extended.

If this body succeeds to resolve national issues, it could possibly evolve into a national defence committee and stay in force even after a president is elected, the source said, making clear that army figures could join the group later.

Al-Katatni, whose FJP party has led early polling in recent weeks, said an advisory council would detract from the (parliament) and intervene in the formation of the founding assembly to draft the constitution.

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The new parliament's primary task will be to pick a 100-strong body to write the new constitution, but the army wants its civilian advisory group and the cabinet to help set the guidelines for the ambitious project.

The main goal of this advisory council is to co-opt the forces of the revolution further, said Khalil Anani, a political analyst, adding that the army was keen on protecting its independence from civilian and parliamentary control.

Clashes between police and protesters demanding that the army cede power have killed over 40 people over the last month.

The advisory council has already met Chief-of-Staff Sami Annan for initial discussions on key issues, such as economic aid, which will be determined after parliament convenes.

The advisory council has already discussed and offered its views on U.S. economic and military aid to Egypt ... and to what extent Egypt can forgo economic aid, the source said, refusing to comment on what could happen to the military aid.

Egypt's military receives $1.3 billion (831.8 million pounds) from Washington each year.

Among the 30 members in the advisory group are presidential candidate Amr Moussa and Islamist presidential candidate Mohamed Selim Al Awwa. Abdel Moneim Aboul Futuh, another Islamist candidate running for the presidency, declined to join the body.

The group will convene weekly and meet the military council at least once a month.

(Writing by Marwa Awad; Editing by Crispian Balmer and Ben Harding)