The Hollywood duo - who play ballerinas in the Darren Aronofsky-directed psychological thriller - engage in some memorably X-rated action in which they share a passionate kiss before tearing each other's clothes off.
The jaw-dropping scene in 'Black Swan' topped the list based on a poll of 2,124 British adults, conducted by VouchersCodePro.
Ryan Gosling and Rachel McAdams's love scene in the 2004 romantic drama 'The Notebook' came in second place, while Jake Gyllenhaal and Heath Ledger's scene in 'Brokeback Mountain' came third.
Other iconic movies to feature in the list included 'Titanic', 'Pretty Woman' and 'Dirty Dancing'.
The results of the poll were released shortly after Natalie - who won an Academy Award for her performance in 'Black Swan' - admitted to being thankful that she grew up in an era before social media.
The 34-year-old actress said: ''I was in that lucky window: there was no Twitter, Facebook or Instagram. I went out and got drunk with my friends and no one knew.''
Top 10 UK favourite sex scenes:
1) Natalie Portman and Mila Kunis
2) Ryan Gosling and Rachel McAdams
3) Jake Gyllenhaal and Heath Ledger
4) Leonardo Di Caprio and Kate Winslet
5) Jamie Dornan and Dakota Johnson
'50 Shades of Grey'
6) Richard Gere and Julia Roberts
7) Robert Pattinson and Kristen Stewart
8) Michael Douglas and Glenn Close
9) James Spader and Maggie Gyllenhaal
10) Patrick Swayze and Jennifer Grey
Seven songs to take you to victory, or seven songs to dry your eyes to.
Idris Elba has unveiled his new indie imprint 7Wallace Music.
It's that time of year again. 'Today we celebrate our Independence Day!'
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