Half of the UK’s leading retail web sites are performing worse than a year ago, according to the latest research, with experts warning this could hit firms’ Christmas sales.

Online shopping revenues from the festive period this year are expected to top £10bn. But with a shopping cart abandonment rate of one per cent, retailers stand to lose up to £100m, says the annual benchmark report by internet specialist Webcredible.

Even some big-name retailers are struggling to keep up with online sales growth of up to 40 per cent annually. Some 55 per cent of stores dropped down the rankings compared with last year, largely because of usability problems such as hidden delivery costs, poor product descriptions, confusing check-out procedures, lack of help options or repeated error pages.

The Marks & Spencer site dropped from first to 12th, its usability score plummeting from 81 to 55 out of 100. And Boots dropped 16 points from 67 to 51.

This year’s best performer is HMV, followed by Game ­ which jumped to second from 18th last year.

Major projects that focus on front-end design rather than customer experience are at the root of the problems, according to Webcredible director Trenton Moss.

“With a redesign, usability can be sacrificed, but if customers can’t find what they are after, they will go elsewhere,” he said.

The prevalence of higher-bandwidth connections and Web 2.0 technologies are a factor in design taking precedence over performance, said Jupiter Research vice president Mark Mulligan.

“The rise in rich media means there is a temptation to go down the route of look and feel and neglect substance and usability,” said Mulligan.

“Visitors can be put off by an over-engineered experience and retailers risk losing customer loyalty.”

Marks & Spencer refutes the allegations of declining online performance. Web sales grew more than 60 per cent in 2006-2007, with 55 million visits generating £100m of revenue.

“In March, we launched our new web site which looks fresh and has a range of new features ­ including a facility that lets you search by size, colour, style and price, from more than 50 departments,” said a spokeswoman.