Russians Julia Volkova, 17, and Lena Katina, 18 - known as Tatu - knocked Fame Academy's David Sneddon off the top spot with their single All The Things She Said.
In a raunchy video accompanying the single, the girls kiss and fondle one another while dressed in school uniforms.
The video prompted TV presenters Richard Madeley and Judy Finnigan to call for the song to be banned.
Top 10 singles
Madeley branded them "sick", and Finnigan denounced them as "paedophilic entertainment".
The duo's creator, Ivan Shapovalov, has admitted that Tatu are what he called an "underage sex project" designed to appeal to men in search of "underage entertainment".
The child protection charity Kidscape has reportedly described Tatu as "pathetic".
"Child pornography is not funny and should never be laughed at, it is disgusting," Kidscape's Michele Elliott told the Daily Mail.
Although the teenagers claim to be lesbian lovers, many critics suspect their image is manufactured.
They are said to have boyfriends in Russia.
Bandmate shunted
Elsewhere in the top 40, Destiny's Child member Kelly Rowland has gone straight in at number two with her solo single Stole.
She shunted fellow bandmate Beyonce - whose collaboration with Jay-Z was at number two last week - down to four.
Sneddon's Stop Living The Lie has dropped to three.
New entries include Kelly Osbourne - daughter of rocker Ozzy - with Shut Up at number 12.
In the album chart, Britney Spears's former boyfriend Justin Timberlake maintained his number one position with Justified.
Avril Lavigne's Let Go slipped from two to three, swapping places with Busted.
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