YouTube and social media stars are paid up to £75,000 for a single Facebook post, £67,000 for a video and as much as £53,000 per Snapchat entry by UK marketers hungry for access to their millions of followers.

The majority of marketers expect to see these costs rise over the next year, despite a massive 85% admitting they aren't sure how influencer fees are calculated. A stunning 35% say they expect spending on influencers to increase by over 50%.

Over a third, 38%, of marketers cannot tell whether a particular campaign on an influencers social media platforms increased sales or not.

The data has come from a new study by Rakuten Marketing, revealing how UK marketers work with the most popular users of Facebook, Instagram and Snapchat.

The biggest celebrity endorsers include Kendall Jenner (82.4 million Instagram followers), Hailey Baldwin (10.5m followers) and Zoella (11.9m YouTube subscribers).

Internet stars with millions of followers and those who work in the premium fashion sector can earn up to £160,000 per post, Rakuten claims.

While these startling figures are reserved for celebrities with millions of followers keen to buy whatever they promote, even so-called "micro-influencers" with up to 10,000 followers can expect to be paid £1,500 by marketers for posting on Facebook. YouTube accounts with 10,000 subscribers can earn up to £3,000 for a positive review, unboxing video or hands-on demonstration of a new product.

But just 20% of marketers claim they are able to demonstrate the impact of influencers on sales. Additionally, just 29% of marketers believe the influencers they work with "are entirely concerned whether their content drives sales for the brand."

Interestingly, the factor which marketers say would encourage them to invest more in influencers is "greater transparency and better reporting of influencer contribution to sales". Influencers have in the past been criticised for not clearly stating when they are being paid to promote a product, compared to genuinely describing something they are a fan of.

James Collins, Rakuten Marketing's senior vice president of global attribution, said: "Influencer marketing can be hugely effective but marketers are commissioning expensive posts without understanding the real impact on the purchase journey. It's essential that marketers question influencer fees and use attribution tools to measure the effects of this activity in order to create strong, value-driven relationships between brands and influencers."