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If your income in either of those tax years was more than £100,000 then you lose £1 of your personal allowance for every £2 of income you had in excess of £100,000. The only way you can mitigate such a loss is to pay pension contributions or make gift aid donations in the same tax year. As you probably had a full personal allowance in each of your tax codes for 2017/18 and 2018/19, the loss of some or all of it will give rise to a tax underpayment. The same will apply of you or your spouse or partner received child benefit.
Take a look here for more information on "adjusted net income".
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