As if figuring out mobile advertising wasn’t hard enough in the first place, researchers are now claiming that a huge proportion – as many as 40% – of all mobile ad clicks are completely worthless.

New research by the German mobile marketing company Trademob shows that a disturbingly high four out of ten mobile advert clicks can be put down to accidents, or outright fraud. And even worse, they believe that the number of fraudulent clicks is growing fast. In January of this year, roughly 10% of clicks could be put down to fraud, but over the last nine months it has risen to account for 18% of all clicks.

Trademob, which shared its findings with GigaOM, did admit that it stands to gain from the apparent fraud, as its mobile ad software includes a tracking tool that is able to filter out worthless clicks.

But whether their data is correct or not, it’s clear that click fraud on mobile devices is a big worry for advertisers. Much of the fraud is thought to come from app developers looking to make a few bucks by generating false clicks, says Trademob. One of the most popular ways to do so is to deploy a bot network, which can make it appear as if the clicks are coming from different IPs, making the fraud almost undetectable – although Trademob claims its software can spot them.

Despite being worth around $1.8 billion a year in 2011, mobile ads are posing some tough challenges for advertisers, irrespective of how many clicks are ‘real’ and how many are ‘fake’. The most obvious case is that of Facebook, whose difficulty in making money through mobile platforms is thought to be one of the key factors in its embarrassing stock price slump, but the social media network is not the only one who is suffering. A recent presentation by venture capitalist Mary Meeker shed further light on the grim realities of mobile advertising, revealing that the average cost-per-thousand-ad-viewers (CPM) for mobile ads stands at $0.75, five times less than the CPM of regular internet ads, at $3.50.