'Extremely disappointed' Deputy Prime Minister Nick Clegg said editors were free to publish "what they like".
But Page Three was not "in keeping with the ways in which women both want to be and should be represented", he added on LBC radio.
The Girlguiding's Advocate panel, which campaigns around issues that affect girls, said it was "extremely disappointed".
"A family newspaper should be ashamed of taking such a flippant approach to an issue that is so important to many young people," it said in a statement.