The head of Liberia's election commission, who has been accused of bias by incumbent President Ellen Johnson-Sirleaf's challenger, has resigned, days ahead of a planned presidential run-off vote.

I chose to step down for the sake of Liberia and so that (challenger Winston Tubman's) CDC (party) would not have an excuse not to participate in the run-off, National Election Commission (NEC) Chairman James Fromayan told Reuters on Sunday.

Tubman last week threatened to withdraw from the November 8 run-off, the country's second post-war vote, unless there was a change of leadership at the election commission.

Fromayan, who has denied any wrong-doing, said he would be replaced by Elizabeth Nelson, his No.2, but he said he did not know it would be a permanent arrangement.

There was no immediate reaction from Tubman's camp.

Johnson-Sirleaf won 43.9 percent of the votes in the October 11 election while Tubman, her closest rival, won 32.7 percent.

Newly-named Nobel Peace laureate Johnson-Sirleaf is now strong favourite for the run-off having secured the backing of former rebel leader Prince Johnson, who came third in the poll with about 11.6 percent.

Last week, there was confusion over whether the CDC would take part in the second round of voting, with party officials issuing contradictory messages. Tubman has accused the election body of not taking his complaints seriously.

A row had also broken out over a letter, which was sent by the election commission and wrongly stated Tubman's running mate George Weah had won the majority of the votes in the first round.

The disputes had threatened to derail the country's first locally organised poll since its civil war. The vote will be seen as a bellwether of its progress since 14 years of on-and-off conflict ended in 2003.

The last poll was held in 2005 and foreign mining and oil firms are preparing to pour in billions of dollars to develop resources in the West African state.

(Reporting by Clair MacDougall; Writing by David Lewis; Editing by Sophie Hares)