UK e-commerce sites are jeopardising more than £300m in lost sales each year and risk driving customers to competitors because of 'invisible errors' that can't be detected by web analytics, according to research from website testing experts.

One third of the consumer visits to websites tested by SciVisum experienced more than three per cent error rates, while more than 10 per cent demonstrated extreme inconsistencies in delivery speed of the online journey.

"This is a worrying trend for eCommerce and IT directors and for consumers with Christmas just around the corner. Poor performance and web errors will mean lost sales," said Deri Jones, CEO of SciVisum.

"Companies wanting to maximise their online sales this Christmas need to check the performance of their sites now. Those that fail to do so might as well include a link to their competitors' site."

SciVisum recently investigated the performance of 40 online sites from the retail, finance, insurance and travel sectors over a period of six months. The study confirmed that customers of e-tailers are being exposed to a significant number and range of problems online which prevent them finishing their desired 'journeys'.

"Invisible errors are not outages affecting 100 per cent of users, but are problems that impact a percentage of users at any point in time. A problem that impacts say 1 in 100 random users on a particular journey is not reproducible by IT teams, and so frequently remains unresolved," said Jones.

SciVisum's testing adopts a mystery-shopper approach and visits the site and attempts to make a user journey every five minutes throughout the day. This allows the company to see what customers see and makes it possible to identify a range of intermittent problems that impact real users, but that are invisible to any other analysis.

The most common problems include: session swap – where two users see each others' online sessions; page not delivered errors; jump back – the user is in error forced back several pages; page content incomplete; and shopping basket errors.

The research also highlighted inconsistencies in the delivery speeds of the journeys that users undertake. More than 30 per cent of journeys experienced performance varying by more than 200 per cent, with one in 10 varying more than 300 per cent, data averaging over a seven-day period.