Shall I compare thee to a Summer's day?, asked Shakespeare. Now, women of the world have replied: No, just tell me I have beautiful lips.

A new survey of online flirting by dating website has found that the best Internet chat-up line for men to use is: You have beautiful lips.

We have found the Holy Grail of flirting, said Badoo Director of Marketing Lloyd Price.

The site, which boasts 87 million registered users worldwide, analysed the success rates of opening lines from nearly 200,000 online flirtations in 11 languages over the course of one month to come up with a Compliment Success Index.

Site users were invited to use one of 12 different chat-up lines. Each complimented an aspect of a woman's body or appearance. Researchers measured the success rate of each line in two ways: first, their success at prompting any response at all; second, their success at launching a longer conversation -- one going back and forth at least four times.

Complimenting a woman's lips was the most successful overall, but other lines worked best in particular countries.

American, French, Italian or Brazilian women were more responsive to You dress beautifully. Brits preferred praise for their legs, Spaniards responded to compliments about their hair, while Germans and Canadians preferred skin praise. Dutch and Portuguese women liked: You have beautiful ears.

Jo Hemmings, a British behavioural psychologist said women responded best to compliments about their lips because it's a bold approach that seems more personal.

What many women want is for men to take the initiative and not be wishy-washy, Hemmings said in a statement accompanying the survey. A lot of men on dating sites send a sort of generic message and women recognize something that hasn't been customised for them.

David Givens, American author of the book, Love Signals: A Practical Field Guide to the Body Language of Courtship said the results made sense because women focus a lot of attention on their lips.

From adolescence onwards, they cosmetically adorn their lips, applying lipstick and colours. They have been doing so since ancient Egypt.

(Reporting by Paul Casciato; Editing by Steve Addison)