Avoiding double taxation

Dear HMRC, I have been a tax resident in Britain since 2014. However, the Portuguese tax authority claims that I am a resident in Portugal earning income abroad (Britain), which they are demanding to be taxed in Portugal. I have accumulated a wealth of evidence proving my tax residency in Britain, including official documentation issued by the UK government (Settlement Status, Letter of Confirmation of Residence, etc), all of which has been rejected as evidence by the Portuguese tax authority. They have now contacted HMRC to recover this “debt”, which has already been taxed in the UK. In view of this, I have the following questions: 1) Can I be considered a tax resident, simultaneously, in two different countries, in this case, both the UK and Portugal? 2) If yes, can I be doubly-taxed on my income, despite the existence of a double taxation agreement between Britain and Portugal? 3) If not, and given the fact that the Portuguese tax authority is impervious to evidence, including HMRC official documents, how can I proceed to reclaim my income tax paid in the UK, so that I can hand it to Portugal’s tax authority? 4) If 3 is not an option, what else can I do to avoid being doubly taxed on my income? Many thanks in advance for your reply.

Hi,
We can only provide general information and guidance in this forum.  
For an answer to a detailed question of this nature, you would need to contact our Self Assessment team below, or seek professional advice. 
Self Assessment: general enquiries
Thank you.