KABUL: The grandson of Afghanistan’s last king pulled out of the country’s presidential race on Wednesday and backed a candidate increasingly seen as the choice of outgoing President Hamid Karzai.

The withdrawal of Mohammad Nader Naeem narrows the field for the April 5 poll to eight and hints at further deal-making in the race to lead Afghanistan as Nato troops prepare to exit.

Naeem, whose grandfather King Mohammed Zahir Shah was ousted in 1973 after 40 years of rule, is the third candidate to pull out and the second to endorse former foreign minister Zalmai Rassoul.

Rassoul has also earned the backing of Qayum Karzai – the president's brother – who pulled out earlier this month.

Naeem announced his decision at a gathering in Kabul attended by both Rassoul and Qayum Karzai.

“We have endorsed doctor Zalmai Rassoul's team and will be working together to win the election,” Naeem's spokesman Mustafa Kakar said.

“The decision to join with doctor Rassoul's team was made in a jirga, representing some 2,500 elders from all over Afghanistan.”

Naeem was not seen as a serious challenger in the race to succeed Karzai, who is constitutionally barred from seeking a third term.

The three leading names are Rassoul, Abdullah Abdullah, who came second in 2009, and former World Bank economist Ashraf Ghani.

The new president faces a testing term in office as Nato combat troops depart and the national army and police are left to fight the Taliban insurgency alone, 13 years after the Islamists were ousted from power.

President Karzai has pledged not to publicly endorse any candidate, but observers have suggested he may be at work behind the scenes to help Rassoul.

The Taliban have vowed a campaign of violence to disrupt the election and on Tuesday attacked an Independent Election Commission office in Kabul, killing five people.

Voting on April 5 is likely to be followed by a run-off election in late May between the two leading candidates, with a new president chosen in June or July.