Car insurance customers are being hit with "eye-watering" administration fees, an investigation by Which? has found.
The consumer group, which is running a "stop sneaky fees and charges" campaign, found that the admin fees used by car insurance providers can vary widely, with some charging double the average for certain costs.
Of the 44 insurers Which? looked at between May and August, it found that five did not have any adjustment fees for changing details such as a name or address, while others charged up to £35. The average fee for this was £22.79, Which? found.
Two-thirds of insurers charged for cancelling during the 14-day cooling-off period, with the cost being as high as £75.
While some insurers did not charge cancellation fees for ending the policy early, the average cost was £49.55 - and the fee could be as high as £75, Which? found.
Some insurers charged set-up and renewal fees - to pay for the cost of arranging cover - with costs reaching up to £50.
The average charge for sending out a replacement insurance certificate or a copy of the policy document was £14.18, but some firms charged £30, while 10 insurers did not charge.
Previous Which? research found that two-thirds (68%) of consumers thought companies used separate fees to trick people into thinking the product or service was cheaper than it was.
Which? is urging companies to set out all fees and charges clearly so consumers can easily compare between providers.
It said all fees associated with setting up, amending or duplicating any type of insurance policy should be "reasonable" and no more than the cost to the company.
Which? executive director Richard Lloyd said: "We've found some insurers charging customers eye-watering admin fees that can be hard to avoid, and people often don't know what they are actually paying for. We want companies to ensure their fees reflect actual costs."
James Dalton, director of general insurance policy at the Association of British Insurers (ABI), said: "The motor insurance market is highly competitive and average premiums are lower than they were two years ago with insurers passing on savings of over £1 billion to customers following the recent reforms to tackle whiplash.
"Customers should always read their policy documents carefully and speak to their insurer or broker if they have any questions about any charges for making changes to a policy or for cancelling it. In accordance with the rules of the Financial Conduct Authority and relevant legislation, the fees insurers charge must be clearly and fully set out and broadly reflect the costs they incur."