10:20 AM PT -- Netflix tells TMZ ... "The letter was sent to the DCMS Select Committee on 23 May, well before any legal case was filed, and has been publicly available since. It does not impact our legal position."

Netflix seems to now be backtracking on the "true story" nature of "Baby Reindeer" -- they've reportedly admitted that the "real-life Martha" is not, in fact, a convicted stalker.

According to Deadline ... the streaming giant recently submitted a letter to Parliament's Culture, Media and Sport Committee in which their senior UK director of public policy admitted Fiona Harvey -- the woman who we all now know served as inspiration for Martha in the show -- was not actually convicted of stalking, despite being depicted as such on TV.

If you haven't seen it ... the show ends with an emotional scene where Martha receives a guilty verdict in court and goes to prison -- the character's second conviction, as described/shown in the series.

Fiona herself has hit back at this specific moment in the show ... filing a defamation lawsuit, and insisting she never went behind bars, let alone being found criminally guilty for stalking.

While this is the first we've heard of Netflix copping to dramatizing that section of their show -- even though they advertise 'BR' as being a "true story" -- anyone following closely knows the show's writer, director and star Richard Gadd already said the show deviated a bit.

RG submitted a 21-page long letter to a federal court in L.A. last summer ... in which he reportedly says he never tried to identify Fiona Harvey and admits her personality doesn't line up 100% with Martha's ... and that they added elements to her character to up the tension, etc.

That said, he did reiterate his claim that Fiona stalked him in the 2010s ... and, he says he went as far as getting a First Instance Harassment Warning against Harvey from cops in 2016 -- but, didn't provide evidence for a conviction.

Regardless of this latest admission, Netflix is still standing behind its lawsuit ... they've been insisting Fiona hasn't proven defamation since her reputation, as they see it, was already ruined by past stalking allegations -- including by ex-colleagues/acquaintances besides Gaad.

We've reached out to Netflix about this letter ... so far, no word back.