Tubman said he would not accept the results of the runoff election, which attracted only 37 percent of registered voters in the West African nation, Britain's Guardian reported.
Sirleaf, Africa's first female president and the winner of a Nobel Peace prize, is expected to have a difficult struggle to prove the legitimacy of her government, the newspaper said.
Tubman, the nephew of former President William Tubman, who served from 1944 until his death in 1971, alleged fraud in the Tuesday voting. Election observers, however, said there was no evidence of significant irregularities or systematic fraud.
Notable deaths of 2014 [PHOTOS]
More remains found at MH17 crash site in Ukraine