Paying monthly for car insurance can lead to paying hundreds of pounds extra, according to analysis from a leading consumer group.
Those who pay an annual sum typically incur less cost, research from Which? said.
The group assessed sales data from comparison website GoCompare and found monthly payers were charged £309 more on average than if they paid only on an annual basis.
In a month examined last year, September 2023, it cost £892 on average for a year's cover paid on a monthly basis, while a one-off annual fee was £583.
Roughly a third of car insurance customers paid monthly in 2022, according to data from financial regulator, the Financial Conduct Authority and half of motor insurance holders who have missed any bill payments in the past six months are paying monthly to have their car insured.
The gap between monthly and annual charges has widened, the research showed, as sales on the site were monitored from December 2018 and September 2023.
In December 2018, the gap was £207 as a typical annual price was £460 compared to £667 when charged monthly. This rose to a £251 difference in September 2022, when the monthly price was £738 versus a £487 annual charge.
Which? suggested the difference may be because younger drivers, who typically pay higher premiums, are also more likely to pay monthly as they're less likely to be able to afford an expensive one-off payment.
Those spreading payments over 12 months face additional charges which also bring up prices and interest rates of more than 30% "can be common", Which? said.
Rising costs, such as for energy and repairs, contributed to annual car insurance premiums hitting a new record high, according to industry body the Association of British Insurers (ABI).
In November it said there'd been a 29% leap in paid-for average comprehensive cover during the third quarter of the year, compared to the same period in 2022.