His loss to opposition candidate Adama Barrow, announced by the elections commission last week and followed by his rapid concession, had sparked hope for change in the tiny West African nation following 22 years of Jammeh's authoritarian rule.
However, in a moved that drew widespread condemnation from the international community, the mercurial former coup leader on Friday decried "serious and unacceptable abnormalities" and called for fresh polls.
In a statement broadcast on state television late on Saturday, the Alliance for Patriotic Reorientation and Construction (APRC) said it was preparing a legal challenge to the result.
"The statement of APRC chairman Yahya Jammeh on the 9th December was a prelude of a petition that the APRC is in the process of filing before the Supreme Court against the flawed decision of the Independent Elections Commission," it said.
Under Gambian law, candidates have 10 days starting from the announcement of election results during which to submit a challenge with the court. Monday is a national holiday in Gambia, making Tuesday the final deadline for filing a petition.