Adjusted tax code for mileage relief

In 2019 I was put onto an adjusted tax code to account for tax relief on mileage. Subsequent tax years didn't see a change in my tax code - it has been the same since. I have just had a travel expenses claim denied, and have now been put back to the standard 1257L code, and received a notice that I owe tax for underpayment during the years I was on an adjusted tax code. The agent I spoke to informed me I needed to manually update this every year, and submit a new mileage claim every tax year. Is this correct? Why wouldn't my code automatically default to the standard 1257L if this were the case?

My tax code has now been adjusted to 744L for 2024 tax year for HMRC to claim back the tax I was given relief on during this period. I was advised on the phone I could submit a mileage relief claim for 2020 onwards. If this is approved presumably it will offset the amount I owe, and then my tax code for 2024 will be updated to reflect the mileage relief I was given for 2023? This is quite a mess, and a lot of waiting for responses. I really can't understand why my tax code wouldn't default back to 1257L if it's my responsiblity to submit a new relief claim every year. It just adds further complications to what is supposed to be a simplifed process for claiming tax relief.

Hi Rush787,
If you had relief for expenses in your tax code and the amount of expenses reduced then this would then mean that there would be an underpayment in the current tax year and your tax code adjusted to collect the underpayment. To check if this was the case you would need to contact HMRC Income Tax: general enquiries. Once your claim is reviewed if the expenses are due then your tax code will be updated going forward with the expenses as an estimate for the next tax year. If the expenses change then you will need to contact HMRC.
Thank you.

Thank you for the reply. This is how I understood it - if the expense amount changed then I would need to submit an updated claim. If not, I could leave as-is. This is not what I was told by an agent on the phone - I was simply told that because the expenses "might" change every year, I need to submit a new expense claim. Therefore they have revoked the last 4 years worth of expense relief, and are seeking to reclaim it in the 24/25 tax year. I do not believe this was the correct thing for them to do.

Hi Rush787,
You would need to contact HMRC to check why the expenses have been removed and what information would be needed if expenses were still due for those tax years:
Income Tax: general enquiries
Thank you.