The letter was posted to SNP MP John McNally, who had written to Buckingham Palace on behalf of his mother, Rosa.
Mc Nally told the Mail his mother 'always talked about' how she was born on the same day as the prince.
'To be honest I never thought I would receive a response or at best I might get a polite letter from his office,' he admitted.
Mrs Mc Nally's grandson posted the royal response on Twitter, captioning it 'My gran's 95th birthday. This from someone the same age...'