The 59-year-old triumphed in the Leading Actress category on Sunday evening, beating the likes of Helena Bonham Carter and Anne Reid.
Sarah won for her role as Catherine Cawood in hit BBC drama, Happy Valley, and which saw her have a strong Yorkshire accent.
But as she took to the stage to accept her award, BAFTA viewers were shocked to hear her real voice.
In stark contrast to Catherine’s dulcet tones in Happy Valley, Sarah – who is from Oldham, Greater Manchester, has a polite accent that some described as “posh”.
Taking to social media, one wrote: “Always forget Sarah Lancashire is actually posh…”
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Another added: “Why is Sarah Lancashire putting on that strange accent?”
A third tweeted: “I never knew Sarah Lancashire talked like that in real life!!!”
Meanwhile a fourth shared: “Sarah Lancashire’s accent was a jump scare.”
The actress was very emotional during her acceptance speech at the ceremony, which was held on London’s Royal Festival Hall.
After praising her fellow nominees, she said tearfully: “Thank you Sally Wainwright [Happy Valley writer] I shall forever be grateful to you for this opportunity.”
She then moved on to thank the cast and crew of Happy Valley, which includes James Norton and Siobhan Finneran.
Sarah said: “I feel very, very privileged to have been surrounded by these brilliant actors and I thank each and every one of you.”
Viewers were thrilled over Sarah’s win, after Happy Valley became one of the BBC’s biggest ever dramas.
One wrote: “I was ready to riot if Sarah Lancashire hadn’t won that! Thank God, more than deserved.”
Another added: “The moment of joy we’ve all been waiting for. Seeing Sarah Lancashire crowned Leading Actress .#BAFTATVAwards #HappyValley What a SUPERB performance, what a SUPERB production, cast and crew. What GREAT writing.”
A third tweeted: “Sarah Lancashire – one of my all time favourite actresses! Such talent! She never fails to blow me away – no matter the part! Pair her with Sally Wainwright’s writing and it’s off the scale! Bravo.”
BAFTA TV Awards 2024 winners
Best Supporting Actress
WINNER: Jasmine Jobson, Topboy, Netflix
Best Supporting Actor
WINNER: Matthew MacFadyen, Succession, Sky Atlantic
Best Entertainment
WINNER: Strictly Come Dancing, BBC One
Comedy Entertainment Programme
WINNER: Rob & Romesh Vs, Sky Max
Short Form
WINNER: Mobility, BBC Three
Specialist Factual
WINNER: White Nanny, Black Child, Channel 5
Factual
WINNER: Lockerbie, Sky Documentaries
Reality Series
WINNER: Squid Game: The Challenge, Netflix
Sports coverage
WINNER Cheltenham Festival Day One, ITV1
Male Performance in a Comedy
WINNER: Mawaan Rowan, Juice, BBC Three
Female Performance in a Comedy
WINNER: Gbemisola Ikemelo, Black Ops, BBC One
Limited Drama
WINNER: The Sixth Commandment, BBC One
Current Affairs
WINNER: This World: The Shamima Begum Story, BBC Two
News Coverage
WINNER: Sky News, Israel and Hamas at War
International
WINNER: Class Act, Netflix
Daytime TV
WINNER: Scam Interceptors, BBC One
Soap
WINNER: Casualty, BBC One
Factual Entertainment
WINNER: Celebrity Race Across the World, BBC One
TV moment of the Year
WINNER Happy Valley’s Final Showdown, BBC One
Single documentary
WINNER: Ellie Simmonds – Finding my Secret Family – ITV1