The government has revealed no details but it is widely expected to roll back much of the previous Socialist government's 2010 reform, which gave women the right to abortion on demand up to 14 weeks of pregnancy.
"The draft law will be presented tomorrow at the cabinet meeting," Interior Minister Jorge Fernandez Diaz told reporters on Thursday.
The text "has been handed out to the ministries but I must not and cannot say anything more about its content," he added.
The 2012 reform also allowed women the legal right to abort up to the 22nd week of pregnancy in cases where the mother's health is at risk or the foetus shows serious deformities.
Under the previous 1985 law, abortion was a crime in Spain except in cases of rape, risk to the mother's health or deformation of the foetus.
Justice Minister Alberto Gallardon has indicated that he wants to return to something similar to the law of 1985, but also possibly to go further, curbing abortions in cases of deformation.
Luis Enrique Sanchez, the head of Spain's pro-choice Planned Parenthood Federation, said that "would put these women in a dramatic situation that would create much pain and suffering."
He told reporters in Madrid: "We would find ourselves once again in a situation like in the 1980s when Spanish women had to go to England and France to interrupt their pregnancies."