A shopping experience test considers catalogue variety, plus editorial content, to arrive at an overall rating out of the maximum of five stars.

But a company's overall rating could still vary dramatically depending on the weightings given to the different measures.

It got a paltry 63.4 per cent overall rating in the tests after 13 per cent of owners said they had broken down in the past year.

The reviews are contributing to the overall rating that buyers see on screen, meaning that four-star and five-star items could have had their scores artificially bumped up.

In the previous exercise in 2001 departments were given one overall rating of up to 5*; the new method gives a finegrained breakdown, revealing differences between the very best.