Russia is on its way to formally prohibiting all adoption by same-sex couples and also banning the adoption of Russian children by single people or unmarried couples from countries that allow same-sex marriage.

The lower house of Russia's parliament voted unanimously for the ban on Tuesday.

The bill still needs to be confirmed in a procedural third reading on Friday, then passed by the upper house and signed into law by President Vladimir Putin, but Russia's rubber-stamp legislative system leaves little room for doubt.

Russia has more than 600,000 children who are not in parental custody, including 100,000 who live in institutions.

Adoption has been a major political football since a law was passed last December banning Americans from adopting Russian children.