Issuing the statement in Najaf on Tuesday, Muqtada al-Sadr said he was asking other religious leaders to push for a guarantee of immediate departure of foreign "occupation" troops after the elections.
"Otherwise, our participation will be unlikely," the statement added.
No member of al-Sadr's movement, which earlier was in direct conflict with US troops, features on the main Shia list submitted for candidate registration.
But up to 70 other political parties - most of whom have similarly threatened to boycott elections at various times - have registered, according to Iraq's independent electoral commission on Tuesday.
Election registration update
Commission spokesman Farid Ayar said 79 blocs were at this point set to contest the election, roughly divided among nine coalitions.
Ayar added that the Iraqi Islamic Party (IIP) has presented a list consisting of 275 candidates, which suggested they planned to participate.
The IIP, the country's largest Sunni party, had threatened to boycott the poll unless it was postponed for six months amid relentless violence in Sunni areas.
Political parties and blocs have one day remaining to register their lists for the poll, in which Iraqis are to elect a 275-seat National Assembly charged with drafting a constitution and appointing a new government.
The full lists, with candidates names, will be made public on 20 December - just over a month before the ballot is to be held.