The average cost for annual comprehensive car insurance has dropped to £594.86 – a 9.8% fall from the figure of £659.53 in July last year.
The drop represents the biggest decrease since the AA insurance index began in 1994.
Comparing July 2013 figures with those of April, the AA showed that comprehensive insurance has fallen 3% and third-party insurance is down 2.2%.
Over the last 12 months, car insurance costs have fallen for all age groups, with those aged 23-29 enjoying the biggest decreases with their average costs falling 12.8% to £738.93.
AA Insurance director Simon Douglas said: “This will be welcome news for hard-pressed motorists facing sharp fuel price increases.
“Premiums are falling again thanks to competition, as well as improved fraud detection by the insurance industry and tightening of the law that is beginning to curb the number of spurious new whiplash injury claims.
“The news that hundreds of rogue ‘no-win no-fee’ claims firms have been reined in is welcome and in part, falling insurance premiums reflect that.”