Iraq has signed its biggest oil deal since the US 2003 invasion with Britain's BP and China's CNPC to develop the giant Rumaila oilfield.

The 20-year contract is expected to triple production at the southern oilfield, from the current one million barrels per day (bpd) to around 2.8 million bpd within a six-year period.

"With these contracts Iraq has started a new phase," AFP quoted oil minister Hussain al-Shahristani as saying after signing the $50 billion deal.

He said BP and CNPC would invest $50 billion in the Rumaila oilfield through a joint venture with Iraq's State Oil Marketing Organization, SOMO.

SOMO will have 25 percent, BP 38 percent and CNPC 37 percent of the project.

The deal provoked criticism from a number of Iraqi lawmakers.

An MP on the parliament's oil and gas committee said on Sunday that the lawmakers believe such agreements have to be approved by the parliament beforehand.

The Iraqi government rejects the idea, saying that Cabinet authorization is enough.

Iraqi lawmakers argue that under such circumstances, there would be no guarantee that upcoming Iraqi governments honor contracts signed by the current cabinet.

"We, in the oil and gas committee, are seeking to meet the British ambassador in Iraq and BP officials, to discuss the illegality of the Rumaila oil deal and its detrimental effect on democracy in the country," Jabir Khalifa Jabir, secretary of the oil and gas committee, told Reuters.

Prime Minister Nuri al-Maliki attacked lawmakers on Tuesday, saying this sent the wrong message to those interested in investing in Iraq.

DB/MTM/AKM