"We are very behind in terms of civil rights in Italy," the deputy, Giancarlo Galan, told AFP, saying it would apply the "same rules" as marriage.
Under the proposed law "gay couples will have the same rights and duties as heterosexual ones," he said, particularly on inheritance and pensions.
The bill refers to "homo-affective unions" but avoids the word marriage "because it evokes a sacrality that hurts sensibilities," Galan said.
Italy is now one of very few countries in western Europe not to recognise any form of gay union. Other hold-outs include Greece, Malta and most of the countries in Centraland Eastern Europe.
Galan said he had received support from some members of his party, although Berlusconi himself and the party's secretary Angelino Alfano, who is also the deputy prime minister, have not replied.
"This is a transversal issue. There are people for and against" in the People of Freedom party and the main leftist Democratic Party, he said.
"The climate is rather favourable. In terms of civil rights, Italian society is much more ahead than the political class, which is very afraid of the Catholic hierarchy," he said.
Galan, a former governor of the Venice region in northeast Italy and a two-time government minister under Berlusconi, is an outspoken social liberal.
Aurelio Mancuso, head of activist group Equality Italia, said Galan's move was "positive" but complained that previous draft bills on the subject have never been approved.
But Fabrizio Marrazzo, a spokesman for Gay Centre, said: "The time is right for a law like this to find a large, transversal majority in parliament."
The debate over gay rights in Italy has escalated following France's legalisation of gay marriage and the publication in leftist daily La Repubblica earlier this month of an emotional letter by a gay teenager who said he was considering suicide.