"The JD-U and RJD will merge soon. It is certain and there is no doubt about it," Vashisht Narayan Singh, who is considered close to former chief minister Nitish Kumar, told reporters here.
He said: "The JD-U and RJD will merge and a new party will be formed in Bihar and at the national level."
His remark has come four days after six parties of "Janata Parivar" (Janata family) have decided to unite and authorised Samajwadi Party chief Mulayam Singh Yadav to work out the modalities for the merger.
He claimed that the Rashtriya Janata Dal (RJD) will join the Janata Dal-United (JD-U) government led by Chief Minister Jitan Ram Manjhi after the merger.
"RJD leaders will be inducted into the cabinet," he said.
Last month, it was decided that the JD-U and the RJD will merge ahead of the next assembly polls in 2015.
Nitish Kumar said the RJD and JD-U would contest the assembly polls as an alliance. In August, Lalu Prasad and Nitish Kumar jointly campaigned for the by-election in Bihar. The JD-U, the RJD and the Congress won six of the 10 assembly seats.
That was the first time the two leaders came together after a gap of 20 years. It was in the 1991 Lok Sabha polls that Lalu Prasad and Nitish Kumar last campaigned together.
Lalu Prasad then said that he and Nitish Kumar wanted to send a strong message across the country to unite non-Bharatiya Janata Party (BJP) forces.
Nitish Kumar, who quit as the state chief minister in May after his party was routed in the general elections, has been repeatedly targeting Prime Minister Narendra Modi.
He said Modi has failed to bring back black money stashed abroad within 100 days of coming to power despite the poll priomise.
Nitish Kumar said: "Modi had promised to bring back black money after being elected to power. But he has failed to do that even after 150 days."
The JD-U ended a 17-year alliance with the BJP last year after Modi was declared the prime ministerial candidate of the party.