Image copyright Savage X Fenty Image caption Rihanna models her Valentine's collection for Savage X Fenty

Modern fashion has taught us that the line between the empowerment and the objectification of women is a difficult one for brands to tread.

With PrettyLittleThing coming under fire from the Advertising Standards Authority for their overly sexualised advertising, consumers are more conscious than ever of companies who use sex appeal to sell to young women.

So how lucrative is it to market women's clothing using sex in a post-#MeToo era?

When done right, incredibly.

Take the online retailer TechStyle Fashion Group, for example. Its annual revenue topped $750m (£574m) last year after adding Rihanna's Savage X Fenty label to its range.

The singer's clothing business has made waves in the fashion world by calling for inclusivity, diversity and taking fashion into the streaming age by striking a deal with Amazon Prime.

And the key to its success has been the way it puts female desire at the forefront, says Olga Mitterfellner, fashion marketing and management lecturer at The London College of Fashion.

It's about making women look good in their own eyes, not the eyes of someone else.

"Everything that the brand is doing is actually old school, but sold really well to young people and at the right time," she says. "The female gaze is as old as Delilah, Cleopatra, Agnès Sorel [sometimes called the first official royal mistress] and Madame de Pompadour. But it is great to remind the next generation that they have choices, power and control over who they are and want to be."

Image copyright Savage X Fenty Image caption Rihanna's lingerie line has been celebrated for capturing the female gaze

There has been a movement in fashion toward female empowerment and inclusivity, enabled by social media, she believes.

"The open exchange between individual consumers through social networks has given them a channel to let brands know what they really think and want," she says.

"I would say that people have always wanted inclusivity, but only now have brands found a way to make it financially lucrative on a large scale," she says.

"As with most products out there, fashion brands are selling hope and dreams, which is now the hope of empowerment and inclusivity. But that doesn't mean the customer actually will be empowered, fulfilled and feel included."

She gives two examples to explain how the different approaches work.

"Fenty is marketing underwear with the idea that the woman is in control of her body, her love life and her choices," she says.

"Victoria's Secret for example is marketing underwear with the idea that she can get a man to control her body, her love life and the only choice she has is to look skinny and seductive or else she has no chance in life.

"Both brands promise power through lingerie, though, but one is outdated."

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Victoria's Secret's owner, L Brands, has seen its share price slump as its famously sexualised advertising looked increasingly dated in the modern era. They rebounded sharply in January on the news that its boss (Mr) Leslie Wexner was in talks to step down - raising hopes of a fresh approach.

While the American brand once favoured among young women has notoriously refused to adapt to a new era of female customers, brands like Savage x Fenty, PrettyLittleThing and Missguided have been much more successful.

The key to their success has been simple: listening to customers, says 21-year-old Nottingham Trent University fashion graduate Lucy Legret.

And that doesn't mean shunning sexualised advertising altogether (though many, not just from older generations, might be happy to see less of it).

"I think the younger generation are desensitised to overly sexual advertising, it's all we've ever known," she says.

Image copyright Lucy Legret Image caption Lucy Legret says her generation is desensitised to sexualised advertising

"I think the best thing a brand can do is to understand that feeling 'sexy' is different for every woman. Not every woman feels their most sexy in heels and a tight dress, it's outdated," she says.

"Whatever makes us confident can make us feel sexy."

This sentiment is echoed by Emily, a 21-year-old who works for a fashion magazine. She says that brands can still sell provocative clothing to women - if they do it in the right way.

"I think women definitely value diversity, and want to see themselves reflected in the advertising of fashion brands they are buying from," she says.

"We want clothing that makes us feel empowered, with a focus on what makes us feel good - rather than just looking good for other people."

Emily cites OhPolly as a brand which treads the line carefully between empowering women to feel sexy in themselves, and promoting a specific ideal version of women which they can't ever meet.

"I've shopped at OhPolly a few times for your typical £10 night out bodycon dress, but I prefer to shop at other brands like Topshop, Boohoo or PrettyLittleThing because the clothing choices are a lot more diverse.

"If you look at their Instagram, even though the models are varied in terms of race, they do all look athletic and skinny which doesn't represent all women," she says.

But OhPolly also shows how hard it can be to get it right. It used to have a separate Instagram account for its pictures of plus-size models - a policy it ditched after a serious consumer backlash.

"Some aspects of their advertisement for their Valentine's collection does feel like a late-night adult advert rather than fashion, but I think that if sexuality is framed as something empowering for women, and not just for the gratification of men, then it can still be used effectively to sell clothing," says Emily.

"I don't think brands should shy away from using sexuality to promote clothing. It doesn't have to appear as though it is objectifying women as long as it is marketed in the right way."