If you exceed your ISA contribution limit, HMRC will instruct your provider to repair the account by removing the excess funds and any growth earned on that portion [1]. You then become liable for income tax or capital gains tax on those removed earnings, as they lose their tax-free status [1]. For Cash ISAs, interest on excess may be taxable if it exceeds your Personal Savings Allowance; for Stocks and Shares ISAs, gains or dividends on excess could trigger Capital Gains Tax or dividend tax [2][3]. HMRC does not typically impose a fixed automatic fine for genuine mistakes, but can apply penalties calculated as a percentage of tax lost if the oversubscription resulted from careless or deliberate behaviour [1]. For Lifetime ISAs, any government bonus paid on excess must be returned [3]. Acting promptly to inform HMRC helps avoid additional sanctions [4]. Do not attempt to fix the error yourself; wait for official correction to ensure it is properly recorded.