If you submit your tax returns by paper, the deadline is 31st October.
For online submissions made through platforms such as ourselves, the deadline is slightly further ahead on 31st January.
HMRC have implemented rulings for what tax years can be filed digitally.
If you're submitting an amended return and for arguments sake, you're currently within the 2025/26 submission window (6th April 2026 - 31st January 2027), this would mean you can only submit amended returns digitally for two tax years, the currently one and the previous one:
2025/26
2024/25
If you're submitting an original return (i.e. it's your first time), you may submit up to four years in the past not including the latest filable tax year; so if like above, you're currently within the 2025/26 submission window (6th April 2026 - 31st January 2027), this would mean you're able to submit returns for:
2025/26
2024/25
2023/24
2022/23
2021/22