If you're tax resident in the UK, HMRC expects you to declare your worldwide income — not just income earned here. This applies regardless of whether you've already paid tax on that income in Poland. The UK–Poland Double Taxation Agreement allows you to claim a credit for Polish tax already paid against your UK liability, which often reduces the additional amount owed significantly. HMRC receives financial data from Poland automatically under the Common Reporting Standard (CRS). Polish financial institutions report account data on UK-resident holders to Polish authorities, who share it with HMRC. Undeclared Polish income is increasingly straightforward for HMRC to identify. If this income hasn't previously been declared, coming forward voluntarily is almost always the right move — and results in significantly lower penalties than waiting for HMRC to find it. Not sure if your situation requires a disclosure? Speak to an Expert — one conversation is usually enough to give you a clear picture.