If you are filing on paper it needs to be with HMRC by the 31st October after the end of the tax year. If filing online it is the 31st January after the end of the tax year. Remember to give the post office time to deliver it and to obtain proof of posting.
For example, the deadlines for a 2017/18 tax return are;
Paper, 31st October 2018
Online, 31st January 2019
Tax due must be with HMRC by 31st January 2019.
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