Doubts about referents in the cover letter for a PhD

Hi,

I'm an italian astronomer, and i want to apply for PhD in Netherlands, because one of my colleagues of the astronomical observatory where I produced my thesis work is in contact with a researcher in another university of Netherlands, and he strongly recommended that position.

My advisor recommend that PhD position, too, and other two university in Europe.
Now, my question is: who is my official referent in the cover letter?
My thesis advisor (it's somehow a more important scientist), or my collegue under the other researcher suggestion?

I hope I was clear about my doubts and apologize for the imperfect language.
Thanks

From what I know, your referee should be someone who has been in direct contact with you either in the role of a supervisor or a tutor.

Therefore, your thesis advisor should be your official referee.
Hope that helps.

It is, thank you!