The political movement of Muqtada al-Sadr, a Shia leader, has said it will end a two-month boycott of parliament, signalling an easing of tensions with its Shia allies in the US-backed government.
The Mahdi Army, a militia loyal to al-Sadr, has been identified by Washington as the biggest threat to security in Iraq.
Nuri al-Maliki, the Shia prime minister, has been under pressure to take action against it.
His dependence on al-Sadr's political movement has made that difficult.
'New beginning'
Al-Sadr's group announced a boycott at the end of last year to press for a withdrawal of US troops from Iraq and to protest against a meeting between al-Maliki and George Bush, the US president.
Your Views "One cannot believe that the Shia-Sunni civil war was not pre-planned by the West"Mohammad Karim, Melbourne, Australia Send us your viewsThe al-Sadr movement held a joint news conference on Sunday with members of the Shia Alliance, to which the group belongs, to announce their return to parliament.
"One cannot believe that the Shia-Sunni civil war was not pre-planned by the West"Mohammad Karim, Melbourne, Australia
Send us your views
Bahaa al-Araji, a senior member of the al-Sadr group, said: "Since there has been a response to our demands, we declare that we will attend parliament today."
Mahmoud al-Mashadani, a parliamentary speaker, said that all the parliamentary parties would form a committee to discuss the reasons for the boycott in an attempt to resolve the issues.
"This is a new beginning," he said. "We want to say to the world that an Iraqi solution for Iraqi problems is the key, and others must support these solutions."