Prime Minister Guy Verhofstadt's own Dutch-speaking Liberal Party said it will table a bill to parliament to do away with the legal uncertainty surrounding children in such unions.
"Research and the practical experience of countries which already allow adoption by same-sex and bisexual couple, show that children raised by same-sex parents are not affected in a negative way," the VLD party said in a statement on its Web site.
Parliament last February voted to give gay couples the same legal status as heterosexual ones. It was not immediately clear when the new bill will be introduced.
Three parties opposed that bill - the Francophone Liberals, the far right Vlaams Blok and the Francophone Christian Democrats.
But with help from the Socialists and the Greens, who have always supported the right to adoption, the bill looks set to carry a majority in parliament.
Belgium, whose population of 10 million is 75% Roman Catholic, has increasingly broken from its conservative traditions.
The Vatican has condemned same-sex unions as deviant and a threat to society in a move to halt the growing momentum towards legalising gay marriage in North America and Europe.
Belgium followed the example of the Netherlands which has recognised registered homosexual partnerships since 1998, but it only passed laws allowing them gay couples to marry and adopt children in December 2000.