In a personal Valentine's plea, the 'Happy' star wrote to John Dalli, the European Commissioner for Health and Consumer Policy.
Sent in support of the Humane Society International's CrueltyFree2012 campaign, Lewis's card show's her wearing an End Animal Testing body art message.
"As a compassionate EU citizen who loves both animals and cosmetics, I am sending you my heart-felt plea this Valentine's Day to help animals who suffer to test beauty and personal care products for sale in the European Union," the 26-year-old wrote. "I was so proud when, in 2009, the cruel practice of animal testing for cosmetics was banned in EU laboratories.
"Like millions of other citizens, I was appalled by the needless suffering of such tiny gentle creatures such as rabbits, hamsters and mice. I was delighted that our policy-makers listened to us and introduced not only a testing ban but also a ban on selling cosmetics tested on animals elsewhere in the world from 2013."
She continued: "This sales ban is vital because it means that the EU's strongly held ethical principle that testing cosmetics on animals is morally unjustifiable, would apply not just to its own laboratories but to laboratories across the globe."
However, Lewis - who has been named both PETA's 'Sexiest Vegetarian' and 'Europe's Sexiest Vegetarian' - said that she was "dismayed" to learn that Commissioner Dalli is "considering a proposal that would undermine the 2013 ban by giving cosmetics companies a loophole to continue profiting from animal suffering".
She wrote: "Like most girls, I love make-up, but my ethical principles are also very important to me. No new lipstick, eye-liner or other cosmetics ingredient can ever be worth the suffering and death of defenceless animals forced to endure painful toxicity tests.
"These gentle animals are killed by the hundreds in some tests, despite the fact that manufacturing products free of new animal testing is perfectly possible through the use of thousands of existing, safe ingredients.
"Animals have suffered enough. If their pain has been deemed immoral within the EU, it must also be immoral outside the EU. To trade in their pain would be hypocrisy."
Dame Judi Dench, Ricky Gervais, Ke$ha, Melanie C and Sir Roger Moore are among those who have all joined Lewis, and 140,000 EU citizens, in signing the online petition to save the 2012 sales ban.
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Watch Leona Lewis perform her cover of 'Hurt' at the Royal Variety Performance below: