According to ECOWAS, troops are on alert to intervene in The Gambia if President Yahya Jammeh refuses to step down next month.
Mr Jammeh initially accepted defeat in the December 1 poll, but later said it was flawed.
The ECOWAS chairman said Senegal had been chosen to lead operations “to restore the people’s wishes” if needed.
President Jammeh has already said he will not be intimidated, saying ECOWAS had no authority to interfere.
Mr Jammeh, who has ruled for 22 years, has lodged a case before the Supreme Court to annul the vote after the electoral commission changed some results.
The commission insists the outcome was not affected by an initial error and that property developer Adama Barrow won the poll and should be inaugurated on 19 January.
Marcel Alain de Souza, chairman of the ECOWAS commission, said Mr Jammeh had until that date to comply with its mediators.
“If he is not going, we have stand-by forces already alerted and these stand-by forces have to be able to intervene to restore the people’s wish,” he said.
The Gambia, a former British colony, is surrounded on three sides by Senegal.
“Senegal has been selected by its peers to lead the operations but we do not wish to start a conflict,” Mr de Souza said.
“If he loves his people, he has to be able to negotiate an exit door calmly. If it doesn’t happen, the most radical means will be used.”
Earlier this week, Jammeh lashed out at regional mediators urging him to step down, saying he will not be intimidated.
Nigeria’s President Muhammadu Buhari was appointed chief mediator by ECOWAS.
The Gambia has not had a smooth transfer of power since independence from Britain in 1965.