You do not need to self-report an ISA oversubscription. HMRC identifies these errors automatically through annual contribution data submitted by ISA providers, which are cross-referenced against your National Insurance number [1][2]. Once detected, HMRC will contact you directly and instruct your provider to repair the account by removing excess funds and any growth earned on them [1][2].

You should not attempt to withdraw the excess yourself, as this can complicate the repair process and may invalidate your legitimate tax-free allowance. While HMRC does not typically impose a fixed automatic fine for genuine mistakes, you will become liable for income tax or capital gains tax on the earnings from the ineligible funds [3][4]. Delaying rectification means you accumulate additional tax on interest or gains during that period. For Lifetime ISAs, providers must report breaches to HMRC regardless of your action [4]. The key consequence is backdated tax liability rather than a penalty for non-reporting itself.